Placenticeras meeki, Bohm, 1898

LANDMAN, NEIL H., TSUJITA, CAMERON J., COBBAN, WILLIAM A., LARSON, NEAL L., TANABE, KAZUSHIGE & FLEMMING, ROBERTA L., 2006, Jaws of Late Cretaceous Placenticeratid Ammonites: How Preservation Affects the Interpretation of Morphology, American Museum Novitates 3500, pp. 1-48 : 7-29

Placenticeras meeki / Placenticeras costatum

LOWER JAW

Bearpaw Shale, Alberta

AMNH 47275 is preserved as a flattened steinkern with a film of black material ( figs. 3 , 4 ). It is embedded in a chunk of marooncolored concretion of calcite­ and siderite­cemented silty mudstone. No placenticeratid shell material is preserved in the concretion.

The jaw is subquadrate in overall view, measuring approximately 95 mm in length and 65 mm in width (W/L 5 0.68). The left side of the anterior margin is missing, and the jaw is compressed due to sediment compaction. However, the jaw is weakly convex in cross section, its lateral edges turning obliquely dorsally for a few millimeters.

The anterior margin is narrowly rounded, forming an apical angle of approximately 1058, increasing to a broader angle of 1458 on the lateral margins. There is a narrow slit on the midline approximately 1.5 mm wide at the apex. This slit extends posteriorly 9 mm, narrowing down completely, although the point of the slit is covered. The edges of the slit are sharply defined. At the posterior end of the slit, the wings appear to be joined together, forming a ridge. The slit is bordered on each side by narrow radial ridges, the out­ er margins of which form radial depressions. These flexures are covered in places by patches of coarsely crystalline black material, eliminating most of the relief. The black material consists of finely fractured square and rectangular pieces, resembling a honeycomb, with tiny clear to yellowish crystals embedded in the fractures. The black material in this region represents the thickened portion of the inner lamella.

An elongate patch of golden, finely crystalline material approximately 8 mm long covers the midline starting at the end of the slit. (This material also occurs in patches on adjacent parts of the jaw where it overlies the black material.) This elongate patch is surrounded by coarsely crystalline black material that ends in a lobate outline, representing the border of the outer lamella. The rest of the jaw is partly covered by a thin film of fine­grained black material. There is a low median ridge between the wings but no gap. Starting at the midpoint of the jaw, the two wings are torn apart and become progressively more widely separated. The margins are ragged, probably resulting from postmortem degradation coupled with sediment compaction.

Where the black material is missing exposing the steinkern on the anterior end of the jaw, fine, nearly straight lirae are spaced at intervals of approximately 0.75 mm. The lirae are oriented perpendicular to the midline and slant backward at a slight angle to the anterior margin. The posterior end of the jaw on the right side is covered by thin, irregular ridges that are concave toward the posterior margin. There are also irregular longitudinal creases along the left lateral edge. The surface of the jaw shows bumps and wrinkles suggesting postmortem plastic deformation.

AMNH 47276 is preserved in a fragment of a concretion composed of siderite­ce­ mented sandstone ( fig. 5 ). It is associated with an angular shard of iridescent shell that is probably a fragment of Placenticeras .

The jaw is preserved as a steinkern with patches of black material. It is approximately 85 mm long and 95 mm wide, but it is incomplete (W/L 5 1.12). The right and left wings are each approximately 50 mm wide at their widest points (W/L of each wing 5 0.59). The jaw is convex in cross section, with a gently arched median area and more steeply sloping sides. A small piece of the right anterior portion of the jaw is missing, and part of the posterior region is torn and crushed against a gastropod shell.

The jaw is parabolic in outline. The anterior margin is rounded, with an apical angle of 1458. The apex is slightly projected, with a shallow indentation on each side. A slit is not present at the apex, although the specimen is broken in this area. A groove occurs along the midline in the anterior region, but it is indistinct. It is bordered by two ridges that diverge posteriorly. The ridges develop into broad, raised areas, posterior of which the middle portion of the jaw is not preserved. The jaw ends in a ragged edge.

The material of the jaw consists of three layers. A patch of golden crystalline material makes up the outermost layer and occurs on part of the anterior one­third of the jaw. This material is underlain by a layer of shiny, crackled black material, which in turn is underlain by a layer of more finely textured black material—that is, the dorsal surface of the inner or outer lamella, depending on the position.

Distinct, longitudinal striations parallel the midline and occur at intervals of approximately 0.6 mm on the right wing, especially at its anterior end. They are visible on the layer below the crackled material—that is, on the steinkern—and thus reflect the texture on the dorsal surface of the inner lamella. The posterior end of the right wing also shows regularly spaced longitudinal flexures on the steinkern, but they are broader and more widely spaced than those on the anterior end. In addition, longitudinal creases are present on the lateral margins of the jaw, especially on the right side.

As with the other jaw specimens, the surface of the jaw bears irregularities—for example, a protuberance on the left anterior region—that suggest plastic deformation. The surface is also traversed by raised, meandering, root­like structures, especially on the right posterior region, which may represent postmortem epizoan traces.

AMNH 47277 is an incomplete lower jaw preserved in a fragment of a sideritic, silty mudstone concretion ( figs. 6–8 ). It is not associated with any Placenticeras shell. The specimen is subrectangular in outline and includes most of the anterior and left lateral margins. It is approximately 55 mm long and 50 mm wide (W/L 5 0.91). It is slightly convex in cross section. The posterior portion of the jaw is missing and ends in a ragged edge. Most of the specimen is a steinkern covered in parts with a black layer consisting of shiny granules.

The anterior margin is narrowly rounded with weak indentations on each side of the apex. The apical angle is approximately 1208. The angle expands to approximately 1458 at the transition to the flanks. There is a shallow median depression at the rostral tip that widens to a maximum of approximately 8 mm at a distance of 30 mm from the tip. This depression is bordered by rounded ridges originating 7 mm from the apex. The ridges broaden and diverge posteriorly and are themselves bordered by depressions. Parts of the ridges and depressions are covered with coarsely crystalline black material eliminating some of the relief.

The wings are separated at the apex by a narrow slit 0.5 mm wide. The slit extends 8 mm posteriorly and narrows down completely. However, the edges of the slit are not nearly as distinct as those in AMNH 47275. Posterior of the end of the slit, the jaw is continuous across the midline and consists of a thick layer of coarsely crystalline black material representing part of the inner lamella. At approximately 19 mm adapical of the tip, there is a sharply defined V­shaped outline demarcated by finely crystalline golden material that overlies the black material. This margin represents the junction between the two lamellae and develops into a short median ridge. The outline of the V on the left side extends anteriorly forming a slight curve. The inner and outer lamellae are separated from one another on the sides by an intervening layer of matrix. Posterior of this area, the jaw appears to be continuous across the middle, exposing the ventral surface of the inner lamella (the outer lamella is eroded away). The posterior edge of the jaw is ragged.

The anterior margin of the right wing shows a step­like change in slope 3 mm from the margin and running parallel to it. If the black material were evenly preserved everywhere, this step would not be apparent. The anterior edge of the left wing bends dorsally but is not recurved.

The jaw material is multilayered. The outermost layer consists of scattered patches of golden, finely crystalline material. It overlies a layer of shiny, black, fractured material, with a maximum thickness of approximately 0.3 mm. The black material has a grossly cellular to finely cellular appearance, with colorless to yellowish crystals embedded in it. This layer overlies another layer characterized by irregular small craters and dividing walls, which may represent the impression of beccublast cells (see below).

The tip of the right wing is covered with striations oriented perpendicular to the midline. These striations occur on the steinkern and are spaced at intervals of approximately 0.5 mm. In addition, there is a series of very thin lineations along the anterior margin of the right wing starting approximately 1 mm from the edge and persisting for approximately 2 mm. They occur on the steinkern and parallel the anterior margin. The left edge of the jaw is torn and folded into longitudinal creases. The rest of the jaw shows irregular bumps and wrinkles, indicating plastic deformation.

TMP 87.119.19 ( figs. 9 , 10 ) is a flattened steinkern in a chunk of grayish brown calcareous concretion, retaining a thin film of the same black material observed in the other jaw specimens. Most of the anterior and left lateral portions of the jaw are preserved. As in the other specimens, the middle part of the posterior end is missing.

The jaw is approximately 90 mm long and 70 mm wide but is incomplete (W/L 5 0.78). It is nearly flat in cross section. The left margin of the jaw curves dorsally and bears longitudinal creases. The apical angle is 1408 and the anterior margin shows a slight indentation on each side of the apex.

The apical tip is bisected by a small slit that extends 8 mm posteriorly and then closes up. It is surrounded by an oval area of coarsely crystalline black material, representing part of the inner lamella. Starting 19 mm from the apex, a central depression appears and widens posteriorly; it is bordered by two rounded ridges, which are themselves flanked by divergent grooves. The surface of the jaw is continuous across the middle, possibly representing the ventral surface of the inner lamella.

A thick band of black material 3 mm wide borders the anterior margin. This material is part of the inner lamella and merges into the patch of thickened material at the midline. It is ornamented on the right side with parallel ridges oriented perpendicular to the anterior margin and spaced at intervals of approximately 0.5 mm. These ridges connect up with similar features at the apex that extend at an oblique angle from the midline.

TMP 92.42.21 is a more or less intact left wing and part of the right wing of a lower jaw ( fig. 11 ). It is a steinkern covered with a film of black material and occurs in a fragment of gray calcareous concretion that also contains an elongate piece of Placenticeras shell. The jaw is approximately 85 mm long and 70 mm wide but is incomplete (W/L of the left wing 5 0.78). The anterior margin is nearly straight, and the apical angle is approximately 1608. The anterior edge bends slightly dorsally. The posterior end of the specimen is torn. Three broad flexures on the middle of the left wing parallel the posterior margin and are spaced at intervals of approximately 12 mm.

A small bit of material is missing from the left anterior tip; the right wing tip is more or less intact. The two wings are separated at the apex, but are joined together along the midline by a thin layer of black material starting 3 mm from the apex and extending another 4 mm. The black material reaches a maximum thickness of approximately 0.3 mm on each side of the midline and displays a honeycomb­like texture with elongate white crystals embedded in the spaces between black crystals. The two wings are separated by a narrow, ragged separation, 1–2 mm wide, for the rest of the jaw. This separation is presumably postmortem.

Striations are present on the anterior margin of the jaw where the black material is worn away. They are spaced at intervals of approximately 1 mm and are oriented parallel to the edge, representing features on the dorsal surface of the jaw. In contrast, where the black material is still intact, thin ridges are spaced at intervals of approximately 0.8 mm and are oriented perpendicular to the anterior edge; these features are on the ventral surface of the jaw.

TMP 99.84.2 is a small specimen preserved as a steinkern with a film of black material ( figs. 12 , 13 ). It occurs in a fragment of gray calcareous concretion containing several pieces of iridescent Placenticeras shell. The lateral part of the jaw is missing on the right side and is partially covered on the left. The exposed portion is approximately 60 mm long and 40 mm wide, but it is incomplete (W/L 5 0.67). The jaw is gently convex in cross section and bends strongly dorsally on the lateral margins, where it is covered with longitudinal creases.

The jaw is lozenge­shaped, its anterior margin forming a moderately well­rounded, parabolic outline with an apical angle of 1058. The anterior margin is bent dorsally and bears thin ridges paralleling the edge. The two wing tips are separated along the midline by a slit 1.25 mm wide that narrows down completely 12 mm from the apex. The slit rests on an oval platform of coarsely crystalline black material bordered by radial depressions that broaden posteriorly and extend another 30 mm. The slit ends in a patch of golden­yellowish, finely crystalline material overlying the black material.

Posterior of the slit, the jaw is continuous across the middle and reflects the ventral surface of the inner lamella; the outer lamella is broken away in this area. Part of the outer lamella is preserved approximately 28 mm from the apex. It is separated by a thin layer of matrix from the inner lamella. The outer lamella is continuous across the jaw, but there is an irregular ridge along the midline that persists to the posterior end and is covered with finely crystalline golden material.

UWO.KMC.100125 is preserved in a fragment of grayish­brown, siderite cemented concretion containing numerous shell fragments of Placenticeras ( fig. 14 ). The jaw is approximately 80 mm long and 80 mm wide but somewhat deformed (W/L 5 1.0). The right wing, which is better preserved than the left wing, is approximately 50 mm wide (W/ L of the right wing 5 0.62).

The jaw is parabolic in outline, with a curved anterior margin. The middle of the jaw is nearly flat, and the lateral margins are folded dorsally, with the left margin more strongly folded than the right. Both margins show longitudinal creases.

There is a slit 2 mm wide at the apex of the jaw, which narrows down completely 10 mm from the apex. The slit is not nearly as well defined as in other specimens. It is bordered by narrow radial ridges, partly covered with thick, coarsely crystalline black material, forming an elongate V­shaped area. The ridges are themselves flanked by weakly defined radial depressions that diverge posteriorly. Patches of finely crystalline yellowish material overlie portions of the black material.

Immediately posterior of the V­shaped area, the jaw, which is preserved in this region as a steinkern, is continuous across the midline, reflecting the ventral surface of the inner lamella. However, approximately 13 mm posterior of this point, where the material of the outer lamella is preserved, there is a midline ridge 1 mm high and 1 mm wide, with a central groove. It extends 17 mm posteriorly at which point the two wings diverge and gently curve outward at an angle of approximately 408 from the midline. The ends of the wings are ragged with tears up to 5 mm deep, but if the ends were restored, they would be rounded in outline. An incomplete specimen of what appears to be an upper jaw is preserved in the same concretion, but there is no evidence of a radula.

Pierre Shale, South Dakota

BHMNH 5041 occurs in a carbonate cemented, shaly concretion with specimens of Baculites compressus Say, 1820 ( figs. 15 , 16 ). It is covered with a film of black material and is incomplete but preserves a strong convexity that presumably approaches the original cross section. The jaw is approximately 70 mm long and 60 mm wide (W/L 5 0.86). The maximum width of the right wing, which is more complete than the left wing, is approximately 45 mm wide at its widest point (W/L of the right wing 5 0.64).

The jaw consists of two symmetrical wings that are joined together along the midline but are progressively more torn apart toward the posterior end. The posterior margins of the two wings show irregular edges. Small fragments are also missing from the sides of the wings. In contrast, the anterior margin of the jaw is nearly complete.

The apex is broadly rounded with an angle of approximately 1458. The anterior margin shows a slight indentation on each side of the apex. There is a dramatic increase in the thickness of the black material, manifested as a step­like feature demarcating the boundary of the outer lamella. This feature does not parallel the anterior margin but runs at a slight angle to it so that this feature occurs at approximately 5 mm from the anterior margin near the midline and merges with it on the lateral edges.

A narrow slit 2 mm wide appears at the apex and extends 15 mm before closing up completely. This slit rests on a platform of coarsely crystalline black material, forming an elongate disc. The platform is bordered by two grooves that are nearly parallel, demarcating the boundary of the outer lamella. Immediately posterior of the disc, the jaw seems continuous across the middle, but in a few millimeters a midline ridge of yellowish material appears with the two sides of the wings raised up against it.

The black material of the jaw is similar to that of the other jaws examined and is characterized by a shiny, honeycomb­like texture. Thin, subvertical, curved walls create cells that sometimes contain yellowish colored crystals.

BHMNH 5454a and 5454b occur together in a single gray limestone concretion ( figs. 17–19). They are associated with many specimens of Baculites compressus but no obvious pieces of Placenticeras . BHMNH 5454a ( figs. 17A, 18 , 19B ) is well preserved and composed of black material. It retains its original convex shape and is approximately 35 mm long and 35 mm wide (W/L 5 1.0). The left wing, which is more complete than the right wing, is 25 mm wide at its widest point (W/L of the left wing 5 0.71).

The apex of the jaw forms a slightly point­ ed projection with an angle of approximately 1608. The anterior margin is weakly concave on each side of the apex. A narrow slit initially 1.5 mm wide extends approximately 10 mm along the midline, and develops into a solid ridge at its posterior end. This slit rests on a raised area composed of coarsely crystalline black material. This area is demarcated by narrow grooves, indicating the margin of the outer lamella, creating a V­shaped island. The outer lamella extends to the anterior edge, where it merges with the inner lamella.

BHMNH 5454b is part of a lower jaw in the same concretion as BHMNH 5454a ( figs. 17B, 19A ). It is a steinkern covered with a film of black material. It is convex and approximately 40 mm wide and 40 mm long (W/L 5 1.00). The right wing, which is more complete than the left wing, is approximately 30 mm wide at its widest point (W/L of the right wing 5 0.75). The jaw is subquadrate in overall shape. The anterior margin is well preserved, with a slight indentation on each side of the apex. A step­like feature is present near the anterior margin and forms a slight concavity with it.

Although the apex is broken off, enough of it remains to reveal a narrow median slit, which extends approximately 6 mm. This slit is surrounded by a raised platform of coarsely crystalline black material with a honeycomb­like texture. This material represents the inner lamella and is bordered on the right side by a thick ridge representing the margin of the outer lamella. The outer lamella forms the step­like feature on the anterior margin. On the left side, the outer lamella has broken off revealing a radial depression next to the central platform of black material. An intervening layer of matrix separates the two lamellae.

BHMNH 5456 is part of a lower jaw, with the posterior one­half broken away ( figs. 20 , 21 ). It is in a concretion full of mollusks, including cephalopods that retain aragonitic shells. The jaw is convex and is approximately 55 mm wide at its widest point.

The anterior margin is rounded with an apical angle of approximately 1408. The margin is slightly indented on each side of the apex. There is a step­like feature that forms a slight concavity with respect to the anterior edge. The apex displays a narrow slit 7 mm long, which is replaced by a narrow ridge that extends another 4 mm before disappearing altogether. The slit/ridge is surrounded by an elongate boss of coarsely crystalline black material with clear to yellowish crystals embedded between the black crystals. This central elevation is bordered by grooves that demarcate the margin of the outer lamella. The grooves are filled with bits of black material, but the most striking aspect is the presence of clear to yellowish crystalline struts that cross the grooves at nearly right angles to the midline.

The outer lamella forms the characteristic V­shaped outline around the central boss and the step­like feature on the anterior margin. The outer lamella bears a midline ridge posterior of the V­shaped area. On part of the right side of the specimen, there is a tan crystalline layer that is weathered and pockmarked. It is approximately 1.2 mm thick and lies on the surface of the outer lamella. It is probably diagenetic in origin.

The jaw is broken 27 mm from the apex and reveals a layer of matrix between the outer and inner lamellae ( fig. 20 ). This suggests that the two lamellae are distinct at this point but are fused together on the lateral margins.

UPPER JAW

Bearpaw Shale, Alberta

UWO.KMC.100126 is an incomplete upper jaw preserved in a fragment of reddish brown, siderite cemented concretion ( fig. 22 ). The specimen is approximately 95 mm long and 50 mm wide, with the left wing sharply folded, and most of the posterior part missing. The jaw is U­shaped, with the anterior margin parabolic in outline. As in the lower jaws, the upper jaw consists of coarsely crystalline black material.

The hood area is approximately 25 mm long and shows some thickening in the apical region, although the tip is broken. In dorsal view, the hood is strongly convex, both longitudinally and transversely, nearly forming a dome. Two well­defined radial flexures originate at the apex and diverge posteriorly at an angle of 708, each eventually fading into the longitudinal axis of one of the wings.

Posterior of the thickened apex, the hood is preserved as a black film forming a relatively flat surface that joins together both wings of the jaw. As indicated by the more or less complete right wing, the wings reach a maximum width of approximately 25 mm. Both wings preserve longitudinal wrinkles and cracks.

Part of what is presumably the lower jaw is preserved in the same concretion ( fig. 22 ). It appears as a layer of coarsely crystalline black material. The fragment is approximately 50 mm long and 80 mm wide but is not well enough preserved for description. There is no evidence of a radula.

AMNH 47278 and 47279 are fragments of what appear to be parts of upper jaws ( fig. 23 ). AMNH 47279 ( fig. 23A ) is better preserved and is approximately 90 mm long and 25 mm wide and occurs in a maroon­colored calcareous concretion. Several large pieces of Placenticeras shell are preserved in the same concretion, although it is impossible to demonstrate that the jaw occurs inside a body chamber. The specimen is probably one of the two long wings that make up the upper jaw. Both ends of it are broken off, but it most likely represents the right wing. It is fairly flat with gently sloping sides and consists of coarsely crystalline black material. Longitudinal striae cover one­half of the specimen along its entire length, and are spaced at intervals of approximately 1.5 mm.

Figures

Fig. 3. Lower jaw of Placenticeras meeki Böhm, 1898, or Placenticeras costatum Hyatt, 1903, AMNH 47275, Baculites cuneatus Zone, Bearpaw Shale, Welling, Alberta, Canada, ventral view, anterior on top. A. Overview showing the ragged posterior margin, 31. B. Close­up of the anterior end showing the apical slit and U­shaped outline of the junction between the inner and outer lamellae (arrow), 33.

Fig. 4. Drawings of AMNH 47275 (see fig. 3). A. Hypothetical cross section near the apical tip. B. Close­up of the anterior end. The cross hatched area indicates coarsely crystalline black material; the lined area indicates finely crystalline yellowish material.

Fig. 5. Lower jaw of Placenticeras meeki Böhm, 1898, or Placenticeras costatum Hyatt, 1903, AMNH 47276, Baculites cuneatus Zone, Bearpaw Shale, Welling, Alberta, Canada, ventral view, anterior end on top, showing the curved anterior margin, 31.

Fig. 6. Lower jaw of Placenticeras meeki Böhm, 1898, or Placenticeras costatum Hyatt, 1903, AMNH 47277, Baculites cuneatus Zone, Bearpaw Shale, Welling, Alberta, Canada, ventral view, anterior end on top. A. Coated. B. Uncoated. Part of the coarsely crystalline black material is missing exposing two radial ridges flanked by grooves (arrows), which are expressed as grooves and ridges, respectively, on the dorsal side, 31.

Fig. 7. Close­up of the anterior end of AMNH 47277 (see fig. 6). A. Coated. B. Uncoated. The arrow indicates the edge of the outer lamella, 33.

Fig. 8. Drawings of AMNH 47277 (see figs. 6, 7). A. Hypothetical cross section near the apex. B. Hypothetical cross section approximately 20 mm from the apex. The two lamellae are separated on each side of the midline but fused together along the lateral margins. C. Anterior end showing the V­shaped edge of the outer lamella. Texture as defined in figure 4.

Fig. 9. Lower jaw of Placenticeras meeki Böhm, 1898, or Placenticeras costatum Hyatt, 1903, TMP 87.119.19, Baculites cuneatus Zone, Bearpaw Shale, Welling, Alberta, Canada, ventral view, anterior end on top. A. Overview showing diverging ridges on the anterior end. The arrow indicates the position of the apex, 31. B. Close­up of the anterior end with a median slit at the apex, 32.9.

Fig. 10. Drawing of the anterior end of TMP 87.119.19 (see fig. 9) showing the thickened area of black material along the margin, with narrow ridges oriented perpendicular to the edge. Texture as defined in figure 4.

Fig. 11. Lower jaw of Placenticeras meeki Böhm, 1898, or Placenticeras costatum Hyatt, 1903, TMP 92.42.21, Baculites cuneatus Zone, Bearpaw Shale, Welling, Alberta, Canada, ventral view, anterior end on top. A. Overview showing most of the right wing. The arrow indicates the position of the apex, 31. B. Close­up of the anterior end, 33.

Fig. 12. Lower jaw of Placenticeras meeki Böhm, 1898, or Placenticeras costatum Hyatt, 1903, TMP 99.84.2, Baculites cuneatus Zone, Bearpaw Shale, Welling, Alberta, Canada, ventral view, anterior end on top, 31.

Fig. 13. Drawing of TMP 99.84.2 (see fig. 12) showing the anterior area. Texture as defined in figure 4.

Fig. 14. Lower and upper (?) jaws of Placenticeras meeki Böhm, 1898, or Placenticeras costatum Hyatt, 1903, UWO.KMC.100125, Baculites cuneatus Zone, Bearpaw Shale, Welling, Alberta, Canada. The lower jaw shows a bilobate posterior margin (toward bottom of photo). The left arrow indicates the position of the apex of the lower jaw. The right arrow indicates a fragment of the upper jaw (?), 31.

Fig. 15. Lower jaw of Placenticeras meeki Böhm, 1898, or Placenticeras costatum Hyatt, 1903, BHMNH 5041, Baculites cuneatus Zone, Pierre Shale, Meade County, South Dakota. A. Ventral view, anterior end on top, showing bilobate posterior margin. The specimen retains a strong convex curvature, presumably similar to that in life, 31. B. Right lateral view with the anterior end (arrow) to the right. The anterior margin is weakly indented and the apex is slightly projected, 31. C. Anterior view, 31. D. Close­up of the anterior end showing the outline of the outer lamella (arrow), 33.1.

Fig. 16. Drawing of BHMNH 5041 (see fig. 15) showing the thickened area of black material on each side of the midline slit. Texture as defined in figure 4.

Fig. 18. Lower jaw of Placenticeras meeki Böhm, 1898, or Placenticeras costatum Hyatt, 1903, BHMNH 5454a (see fig. 17). A. Left lateral view with anterior end to the left. The arrow indicates the step­like feature near the anterior edge, 33. B. Overview showing the V­shaped outline (arrow) representing the margin of the outer lamella, 33.

Fig. 19. Drawings of BHMNH 5454a and 5454b (see figs. 17, 18). A. Hypothetical cross section through BHMNH 5454b approximately 10 mm from the apex. The two lamellae are separated on each side of the midline. B. Close­up of the anterior end of BHMNH 5454a showing the junction of the inner and outer lamellae, which is visible due to weathering away of part of the inner lamella. Texture as defined in figure 4.

Fig. 20. Lower jaw of Placenticeras meeki Böhm, 1898, or Placenticeras costatum Hyatt, 1903, BHMNH 5456, Baculites cuneatus Zone, Pierre Shale, Meade County, South Dakota, ventral view, anterior end on top. The upper arrow indicates the edge of the outer lamella. The lower arrow indicates the separation between the two lamellae, fortuitously exposed at a break in the jaw, 32.7.

Fig. 21. Drawing of BHMNH 5456 (see fig. 20). A. Hypothetical cross section (restored) near the apex. B. Close­up of anterior end showing an elongate boss and narrow ridge along the midline. The horizontal lines indicate the crystalline struts that cross the grooves. Texture as defined in figure 4.

Fig. 22. Upper jaw (right arrow) and lower jaw (?)(left arrow) of Placenticeras meeki Böhm, 1898, or Placenticeras costatum Hyatt, 1903, UWO.KMC.100126, Baculites cuneatus Zone, Bearpaw Shale, Welling, Alberta, Canada. The ventral surface of the upper jaw is exposed with the anterior end on top. Most of the right wing is preserved. Part of the lower jaw (?) is visible on the left side, 31.

Fig. 23. Parts of upper jaws (?) of Placenticeras meeki Böhm, 1898, or Placenticeras costatum Hyatt, 1903, 31. A. AMNH 47279, Baculites reesidei Zone, Bearpaw Shale, Travers Dam, Alberta, Canada. B. AMNH 47278, Baculites cuneatus Zone, Bearpaw Shale, Welling, Alberta, Canada.