Thecidellina blochmanni Dall, 1920

(Fig. 3 A–O)

1920 Thecidellina blochmanni Dall, 1920 —p. 283.

1939 Thecidellina blochmanni Dall, 1920 —Helmcke, p. 216, 233, fig. 231. 1965 Thecidellina blochmanni Dall, 1920 —Elliott, p. H858, fig. 742, 3a–b.

1970 Thecidellina australis blochmanni —Pajaud, p. 245, fig. 110 and pl. II, fig. 10. 1973 Thecidellina blochmanni Dall, 1920 —Cooper, p. 8, pl. 8, figs 27–30.

2003 Thecidellina blochmanni Dall, 1920 —Lee and Robinson, p. 354.

2005 Thecidellina blochmanni Dall, 1920 —Lűter and Sieben, p. 188.

2013 Thecidellina blochmanni Dall, 1920 —Logan and Bitner, p. 166, fig. 4A–L.

Diagnosis. Cardinal process of dorsal valve with prominent median keel, large diductor scars with elevated median ridge, massive calcitic formations (“bouffant-style”) in anterior part of brachial lobes.

Type locality. Flying Fish Cove, Christmas Island, Indian Ocean (10º 25’ S 105º 40’ E). Collected at a depth of 84.1 m by C.W. Andrews in 1909.

Suggested neotype. Mature adult shell, ZMB Bra 205, Fig. 3 A–E, dimensions: LV 4.8 mm, LD 3.9 mm, W 4.2 mm.

Figured topotype. Juvenile specimen, ZMB Bra 204, Fig. 3 F–J, dimensions: LV 3.9 mm, LD 2.9 mm, W 3.3 mm.

Additional figured material. WAM Z11798, Fig. 3 K–O, dimensions: LV 7.0 mm, LD 5.4 mm, W 6.4 mm.

Etymology. Named by Dall (1920) after F. Blochmann, Tübingen, who sent a single specimen collected by C.W. Andrews from Christmas Island to Dall for identification.

Description. Shell small for genus, length of ventral valve rarely exceeding 5mm, width about 3.5mm, shell usually longer than wide, endopunctate, anterior commissure rectimarginate, fibrous secondary shell restricted to cardinal process, teeth and tubercles. Ventral valve cemented to substrate by small protegulum. Interarea flat (planodeltidium of Logan and Baker 2013), finely striated with parallel growth lines, hinge teeth prominent, triangular, hemispondylium fused to valve floor, no supporting septum, prongs apically pointed, medially fused, but not usually intact. Dorsal valve smaller than ventral valve, usually elongate; cardinal process bilobate, with prominent central median keel (Figs. 3 A, C) and adjoining diductor muscle scar at the termination of the cardinal process (Fig. 3 C), dental sockets on either side of cardinal process (Fig. 3 H); calcitic pole attached to bridge and cardinal process (Figs. 3 C, D, H. I), massive with flat sides in adults (Figs. 3 C,D); median septum variable in width, arched in juveniles (Fig. 3 G), diverging anteriorly, flat or slightly crested posteriorly, changing to concave anteriorly; brachial cavities deep, floored by occasional tubercles, usually found uncovered but sometimes with delicate canopy where preserved, rows of heavy ridges radiating towards the center (Fig. 3 K) or with massive bouffant-like calcitic aggregations of shell in the anterior part of the brachial lobes (Figs. 3 F,K); peripheral rim with tubercles (Fig. 3 G).

Type material. Lüter and Sieben (2005, p. 188) have outlined the history of this species, culminating in the loss from the USNM of the sole specimen, the holotype (USNM 227822), described but not figured by Dall (1920) and figured but not described by Cooper (1973). In our work on Recent Thecidellina it has become increasingly important to establish a standard for T. blochmanni, hence this move to select a neotype from one of the 18 established topotypes collected by C.W. Andrews in 1909 from Flying Fish Cove, Christmas Island. Twelve of the original topotypes are still in the NHM but the other 6 were subsequently sent to Blochmann at the Zoological Institute, University of Tübingen, Germany, who in turn sent one to Dall for identification. Topotype ZMB Bra 205 (Fig. 3 A–E) from the type locality is here suggested as the neotype of Thecidellina blochmanni to replace the missing holotype, with topotype ZMB Bra 204 (Fig. 3 F–J) illustrated for comparison.

We have also figured a specimen from nearby West White Beach (WAM Z11798) (Fig. 3 K–O) collected by L. Marsh (18.02.1987) from 29m depth at station 11 on the Western Australian Museum Expedition to Christmas Island attached to the underside of the coral Leptoseris (Marsh and Fromont, 2000) . Because of the close proximity of the two localities (see Fig. 4) and the similarity of Dall’s description with the WAM specimens, it is tempting to identify them as T. blochmanni but there are the following differences which may or may not be considered as minor:

1 Ovarian notches are usually absent in older specimens.

2 The calcitic spine is reduced to a thin pole (compare Fig. 3 D, I and N).

3 The internal median keel in the cardinal process is more pronounced.

4 The diductor muscles attachment site on the cardinal process is enlarged, with a prominent median keel. 5 The WAM material is appreciably larger than that from Flying Fish Cove.

The neotype chosen corresponds quite well with the only available image of the holotype in Cooper (1973, pl. 8, figs 27–30). Comparison of the West White Beach (WWB) material with the original description of the holotype by Dall from the Flying Fish Cove (FFC) reveals that ovarian notches are not present in the former, but the internal median keel of the cardinal process is present and well-defined, as it is in the FFC material. The dimensions of the WWB specimens are appreciably larger than those from FFC. In conclusion we err on the side of caution in labelling the WAM material as Thecidellina cf. blochmanni pending possible molecular analysis of specimens from both localities in the future.