144 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 



differs much from that which subsequently the full-grown specimen assumes, 

 being much more oblique, and having the posterior end much more expanded 

 comparatively to the rest of the shell. (PL XVIII, figs. 34 and 35.) 



6. Naiadites obesa, R. Etheridge, jun., 1878. Plate XIX, figs. 1 — 16. 



Anthracoptera ? obesa, B. Etheridge, jun. Quart. Journ. Geo!. Soc, vol. xxxiv, 



pp. 12, 13, pi. i, figs. 12, 13 (and 

 14?), 1878. 

 Anthracoptera tumida, B. Etheridge, jun. Memoirs of the Geo]. Surv. Scotland, 



Expl. of Sheet 31, p. 82, 1879. 

 Mtalina modioliformis, Kirkby, Quart. Journ. Geol. Soc, vol. xxxvi, p. 585, 



1.880. 

 Anthracoptera obesa, Hind. Ibid., vol. xlix, p. 255, 1893. 



— tumida (pars), Hind. Ibid., vol. xlix, p. 255, 1893. 



Non — — Ibid., vol. xlix, p. 255, pi. vii, figs. 13, 14. 



Specific Characters. — Shell tumid, transversely triangular, with a strong diagonal 

 blunted gibbosity or ridge dividing the valves into two nearly equal diagonal 

 halves, more pronounced in the left valve. The anterior end is tumid but small, 

 obtusely pointed, marked off by a very shallow and oblique constriction anterior 

 to and parallel to the ridge. The anterior border is bluntly curved, pointed above, 

 soon sloping downwards and backwards, so that it passes gradually into the 

 inferior border, which in front is oblique but straight, posteriorly is bluntly 

 rounded or even angulated. The byssal notch, most marked in the left valve, is 

 situated just anterior to the point where the oblique swelling becomes marginal. 

 The posterior end is obliquely truncated from above downwards and backwards, 

 making above an obtuse angle with the posterior end of the hinge-line, below 

 becoming rounded off into the lower border. 



The hinge-line is straight, somewhat less than the greatest antero-posterior 

 length of the shell. The umbones are obtuse, delicately pointed at tip, which is 

 situated al a point distant from the anterior extremity of the hinge-line equal to 

 one tenth of the length of that line, not contiguous, raised above the hinge-line. 

 Umbonal swelling is not marked off from the gibbose anterior part of the shell, 

 posteriorly it is continuous with the oblique diagonal ridge, which is very marked 

 in its upper two thirds and best defined on its anterior side; posterior to the ridge 

 the shell becomes gradually compressed, more especially upwards where the 

 posterior slope is convex. The ridge is almost median, but really slightly anterior 

 to the \v\w diagonal of the shell. 



