Axinodon ellipticus Verrill & Bush, 1898

Oliver, P. Graham, 2012, Taxonomy of some Galeommatoidea (Mollusca, Bivalvia) associated with deep-sea echinoids: A reassessment of the bivalve genera Axinodon Verrill & Bush, 1898 and Kelliola Dall, 1899 with descriptions of new genera Syssitomya gen. nov. and Ptilomyax gen. nov., European Journal of Taxonomy 12, pp. 1-24 : 5

Axinodon ellipticus Verrill & Bush, 1898

Axinodon ellipticus Verrill & Bush, 1898: 796 , pl. XC figs 5, 6; pl. XCII, fig. 1.

Axinodon symmetros – Aartsen 1996: 30, fig. 5.

Material examined

Holotype

1 shell, North Atlantic , off New Jersey, United States Fish Commission Albatross, stn 2096, 39˚22'20"N 70˚52'20"W, 1451 fathoms (2864 m), United States National Museum- USNM35175 .

Redescription of holotype ( Fig. 1 )

Shell small, length 3.5 mm, height 3.3 mm. Thin, fragile. Equivalve. Inequilateral, beaks behind the midline. Umbos prominent, beaks prosogyrate. Outline obliquely subcircular, distinctly expanded anteriorly; anterior dorsal margin short, as a poorly defined lunule; posterior dorsal margin indistinct sloping into broadly rounded posterior, anterior broadly rounded more so than posterior, ventral margin broadly rounded no distinct junctions with lateral margins. Sculpture of dense, fine commarginal ridges most obvious on margins. Prodissoconch II distinct, 754 µm across, with weak commarginal lines ( Fig. 1H ). Hinge plate narrow, ligament deeply sunken on a groove running from under the beaks posteriorly for about one-third of the posterior dorsal slope. Right valve lacking any projecting teeth, hinge plate slightly and irregularly thickened below lunule. Left valve with a short, weak. marginal flange beneath the lunule; a very weak protuberance is visible below the beak. Adductor scars oval, roughly of equal size; pallial line entire. Interior with feeble radial striae.

Differential diagnosis

At the generic level Chavan (1969) lists Kelliola Dall, 1899 as congeneric with Axinodon and assigns them to the Galeommatoidea in the family Montacutidae . This is despite Verrill & Bush (1898) describing Axinodon as edentulous and Dall (1899) describing Kelliola with teeth. The hinge of A. ellipticus has been examined carefully to ascertain if it is edentulous as stated by Aartsen (1996) or if a cardinal peg was present but has been broken off. The SEM images presented here ( Fig. 1A ) show no indication of a broken tooth confirming the edentulous condition. Comparisons with figures in Chavan (1969) should not be made as these incorrectly show a shell that is expanded posteriorly and teeth that are not shown in the accurate figures made by Verrill & Bush (1898). From the description below, Kelliola is seen to have a cardinal peg in the right valve ( Fig. 2A ) and therefore Axinodon and Kelliola are not congeneric. In outline, Axinodon looks like Kelliopsis Verrill & Bush, 1898 but the latter has small, distinct teeth ( Verrill & Bush 1898). Verrill & Bush (1898) placed Axinodon in the Thyasiridae as did Aartsen (1996) and Coan et al. (2000), and there are similarities with the thyasirid genus Mendicula Iredale, 1924 . Without anatomical data, it is not possible to confirm the affinity with the Thyasiridae but it is, however, very doubtful that Axinodon is a galeommatid as the latter all display some degree of dentition in the right valve.

Figures

Fig. 1. Holotype of Axinodon symmetros Verrill & Bush, 1898, USNM 35175. A-B. SEM of hinges of right and left valves. C-D. SEM of internal of right and left valves. E. Photo micrograph of internal of right valve. F-G. SEM of external of right and left valves. H. SEM of prodissoconch.

Fig. 2. Holotype of Kellia symmetros Jeffreys, 1876, USNM 170626. A-B. SEM of hinges of right and left valves. C-D. SEM of internal of right and left valves. E-F SEM of external of right and left valves. G-H. photo micrographs of internal and external of right valve.