ANTHRACOMYA DOLABRATA. 93 
2. ANTHRACOMYA DOLABRATA (Sowerby). Plate XIII, figs. 4—9 and 11. 
Unto potasratus, Sowerby. Trans. Geol. Soc., ser. 2, vol. v, pt.3, 1840, pl. xxxix, 
fig. 9. 
— — Brown. Foss. Conch., 1849, pl. Ixxxviii, fig. 17. 
ANTHRACOMYA DOLABRATA, Salter. Geol. Surv. Mem. Iron Ores of S. Wales, 
1861, p. 280, no fig. 
— — Hind. Quart. Journ. Geol. Soc., p. 261, pl. viii, 
1893, figs. 9, 9a, 10a; pl. ix, figs. 1, 1 a. 
Non — a — Ibid., pl. is, figs. 4 and 5, 
Specific Characters.—Shell tumid, subquadrate, transversely elongated and 
expanded upwards posteriorly. The anterior end is short, compressed, and 
comparatively narrow, its upper border being far below the umbones. The 
border is sharp and bluntly rounded, passing insensibly below into the lower 
border, which is rapidly produced downwards and backwards, being almost 
straight at first, but becoming bluntly rounded into the posterior border behind. 
The posterior border is obliquely truncate in its upper part from above downwards, 
rounded below. The hinge-line is straight, much raised posteriorly, equal to 
about five-sixths of the length of the shell in extent. The umbones are obtuse 
and gibbose, raised above the hinge-line, contiguous, situated at a distance equal 
to about one-third of the length of the hinge-line from the anterior end. The 
lunule is long and narrow, the shell is obliquely swollen, the tumidity being 
continuous with the umbones, and gradually rounded off both in front and behind. 
Just anterior to the swelling is a very shallow depression, which becomes wider as 
it progresses towards the lower margin. Posterior to the swelling the shell is 
gradually and regularly compressed into the border, particularly so above, to such 
an extent that the posterior slope of the shell is concave. The tumidity is most 
developed above the widest part of the shell, being just below the level of the 
hinge-line ; downwards and backwards the swelling gradually becomes less, but it 
can be traced to the inferior margin some little way anterior to the posterior 
inferior angle. 
The interior has not yet been exposed. 
Heterior.—The shell is ornamented by fine striz and lines of growth, which, 
crowded in front, become slightly separated as they pass backwards over the shell, 
and on reaching the oblique swelling they become deflected upwards to terminate 
in the superior border. Periostracum thin and wrinkled. 
The ligament is external and small, but erect, equal in length to about one- 
fourth the length of the hinge-line. Posterior to the ligament there is a shallow, 
