Dilemma inexpectatum ( Crozier, 1966 ) Crozier, 1966

Leal, Jos Ẽ H., 2008, A remarkable new genus of carnivorous, sessile bivalves (Mollusca: Anomalodesmata: Poromyidae) with descriptions of two new species, Zootaxa 1764, pp. 1-18 : 12-15

Dilemma inexpectatum ( Crozier, 1966) new combination

( Figures 41–45 )

Corculum inexpectatum Crozier, 1966 , p. 42, figs. 10–12; Powell, 1979, p. 413, fig. 109; ter Porter, 2005, p. 6.

Diagnosis. Shell with sculpture of strong, regular commarginal grooves on posterior surface, with interspaces twice as wide. Byssal gap absent. Escutcheon relatively large, smooth, well-defined, separated by prominent groove from remainder of posterior shell surface.

Description (based on holotype, a left shell valve). SHELL ( Figures 41–45 ): Sculpture of strong, regular commarginal grooves on posterior surface, with interspaces at least twice as wide. Anterior shell surface smoother, with worn aspect and commarginal growth lines that often coincide with grooves on posterior surface. Shell apparently nacreous internally, relatively thick, compressed in anteroposterior direction, cardioid in lateral outline (assuming that right valve is symmetrical with left). Byssal gap absent. Well-developed carina separating anterior from posterior shell surfaces. Umbones and umbonal cavity as in previous species.

Umbones, umbonal cavity, hinge, cardinal-like teeth ( Figures 40, 45 , CT), hinge sockets ( Figure 45 , HS), as in previous species. Posterior lateral tooth (present in right valve only) not seen. Ligament lacking in examined holotype.

Anterior adductor muscle scars ( Figure 45 , AAS), posterior adductor muscle scars as in previous species. Anterior and posterior byssal retractor scars difficult to examine due to worn nature of holotype. Pallial line continuous.

Lunule absent. Escutcheon ( Figure 41 , ES) relatively large, smooth, well-defined, separated by prominent groove from remainder of posterior shell surface.

Periostracum lacking in holotype. Shell color dirty-white with some staining evocative of iron oxide.

Type Material. Holotype, NMNZ M.018488, 3.94 mm (length) × 8.19 mm (height) × 8.16 mm (width; width was calculated by multiplying width of single left valve by two, assuming the shell to be perfectly equivalve), RNZFA TUI, 15 July 1962 (Images available online at Museum of New Zealand Te Papa Tongarewa, 2007.) Paratype, one right valve NMNZ M18491 (not seen; both assumed from type locality).

Type Locality. North of Three Kings Islands, 34º00' S, 171º55' E, 805 m depth, bryozoan and shell bottom.

Remarks. This species is also known only from its type locality. It was originally described as a Corculum (Cardiidae) from the holotype, a left valve (above), and one paratype, a right valve (not examined). The original description gave dimensions as “2.7 mm (length) × 8.0 mm (height).”

Notwithstanding the lack of known soft parts, Dilemma inexpectatum is easily separated from the two new species described above by its sculpture of strong grooves on its posterior surface that contrasts with a relatively large and very well-defined escutcheon devoid of sculpture, a relatively thicker shell, and the lack of a byssal gap on the anterior shell surface.

Figures

FIGURES 34 – 45. Dilemma species. 34 – 40. Dilemma spectralis. Holotype, MNHN 20818, whole specimen and shell views. 34 – 35. Whole specimen. 34. Posterior. 35. Anterior. Left valve is broken adjacent to byssal gape. 36. Right and left valves, lateral. 37. Ligament in posterodorsal view. 38. Detail of posteroventral shell surface showing periostracum with spicules. 39. Detail of anteroventral shell surface showing periostracum with worn spicules. 40. Right valve, ventral view of internal surface showing hinge. Scale bars: Figures 34 – 36 = 5 mm; Figures 37 – 40 = 1 mm. 41 – 45. Dilemma inexpectatum (Crozier, 1966). Holotype, NMNZ M. 018488, left valve. 41 – 44. Valve views, from left to right: posterior, internal lateral, anterior, external lateral. 45. Detail showing cardinal-like tooth. Scale bars: Figures 41 – 44 = 5 mm; Figure 45 = 1 mm. Abbreviations: AAS = scar of anterior adductor muscle; BG = byssal gap; BGA = byssal gape (in mantle of broken shell); CT = cardinal-like tooth; ES = escutcheon; HS = hinge socket; IL = inner ligament layer; OL = outer ligament layer; SM = shell margin. Figures 41? 44 courtesy Bruce Marshall, copyright Museum of New Zealand Te Papa Tongarewa.