148 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 
Myautna crassa, R. Etheridge, jun. “On Carboniferous Lamellibranchiata,” 
Ann. Mag. Nat. Hist., ser. 4, vol. xv, 
p- 427, pl. xx, figs. 1—5, 1875. 
— — var. MoproLirormts, R. Etheridge, jun. ‘Invertebrate Fauna 
of Lower Carb.,’’ Quart. Journ. Geol. 
Soc., vol. xxxiv, pp. 18, 14, 1878. 
— — R. Etheridge. ¥oss. Brit. Paleoz., p. 286, 1888. 
Anruracoprera crassa, Hind. Geol. Mag., dec. 3, vol. x, p. 514, 1893. 
Specific Characters.—Shell transversely produced, large, triangular, massive. 
The anterior end is swollen, for the greater part solid, acutely pointed above, 
forming a small lobe in front of the beaks. The anterior border very thin, and 
showing layers of shell above, slopes rapidly downwards and backwards, being only 
slightly convex at first; the edge of the shell then becomes thin and sinuous, in the 
centre of which is the byssal notch, much more pronounced in the left valve. 
From the byssal notch the lower border is almost straight, but still directed 
obliquely downwards and backwards. The posterior end includes the greater 
portion of the shell, and has a truncate posterior border very slightly obliquely 
cut from above downwards and backwards; it joins the inferior side by a bluntly 
rounded curve, and above forms an obtuse angle with the posterior end of the 
hinge-line, which is straight, only very shghtly shorter than the extreme length of 
the shell. The umbones are anterior, not quite terminal, small, and pointed, very 
slightly elevated above the hinge-line ; in unworn specimens a little incurved at the 
apices so as to project over the hinge-line. From the umbones a well-developed 
oblique gibbosity, more conspicuous in the left valve, extends across the shell 
downwards and backwards towards the inferior border, which it reaches some 
distance in front of the posterior inferior angle of the shell. Anterior to this 
swelling the shellis much constricted, the left valve being the more so. This con- 
striction becomes broader and shallower as it approaches the edge of the shell, 
where it corresponds to the simuosity of the margin and the byssal sinus. 
Posteriorly the oblique gibbosity becomes compressed, especially upwards into 
the hinge-line, so that the shell becomes flat towards the posterior end, and the 
valves must here be almost in contact. 
Interior.—The anterior adductor impressions are double, often triple, pit-like. 
When triple, the middle scar is the largest. The posterior adductor scar is large, 
shallow, very delicately ribbed, situated on the posterior slope some little distance 
within the margin of the shell. The marks for the insertion of the byssal muscles 
are seen just below the hinge-lne. The inner surface of the shell is smooth. 
Its pallial line is some little distance within the valves, marked by a row of pits 
which in some specimens have coalesced into a continuous line. 
The hinge-plate is elongated, broad in front, becoming narrow behind, very 
