148 CARBONICOLA, ANTHRACOMYA, AND NAIADITES. 



Myalina crassa, S. Etheridge, jim. "On Carboniferous Lamellibranchiata," 



Ann. Mag. Nat. Hist., ser. 4, vol. xv, 

 p. 427, pi. xx, figs. 1—5, 1875. 



— var. modioliformis, E. Etheridge, jun. " Invertebrate FauDa 



of Lower Carb.," Quart. Journ. Geol. 

 Soc, vol. xxxiv, pp. 13, 14, 1878. 



— R. Etheridge. Foss. Brit. Palaeoz., p. 286, 1888. 

 Antiiracoptera crassa, Hind. G-eol. 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 slightly 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 shell is 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 sinuosity 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, (lie middle scar is the largest. The posterior adductor scar is large, 

 shallow, xwy 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 

 arc seel) just below the hinge-line. The inner surface of the shell is smooth. 

 Its pallia! line is some little distance within the valves, marked by a row of pits 

 which in some specimens have coaleseed into a continuous line. 



The hinge-plate is elongated, broad in front, becoming narrow behind, very 



