238 CARBONIFEROUS LAMELLIBRANCHIATA. 



diagonal ridge; the straight rectangular posterior side distinguishes it from 

 every species of Cypricardia with which I am acquainted." The word Cypricardia 

 must be a misprint, for the shells figured by M'Coy under this generic name have 

 no resemblance at all to Protoschizodus. 



P. rectangularis resembles P. axiniformis in shape, but is much more gibbose, 

 and has a much more compressed and expanded dorsal slope, and a much 

 stronger oblique ridge. 



I have at present seen only very few examples of this species ; it seems to 

 be rare, but more frequent in Ireland than in England. The shell figured, 

 PI. XVIII, fig. 20, from Settle is in the Burrow Collection of the Woodwardian 

 Museum, Cambridge. The other, fig. 9, PI. XIX, is in the collection of the 

 Geological Survey of Ireland, in the Museum of Science and Art, Dublin. 



Protoschizodus trigonalis, de KonincJc, 1885. PL XVIII, figs. 14, 14 a. 



Peotoschizodus teigonalis, de KonincJc, 1885. Ann. Mus. Eoy. d'Hist. Nat. 



Belgique, torn, xi, Appendix, p. 218, pi. xiv, 

 figs. 9, 10. 



Specific Characters. — Shell of very moderate size, gibbose, subtrigonal, slightly 

 inequilateral, greatest dorso-ventral and antero-posterior diameters almost equal. 

 The anterior end gibbose above and compressed below, projects forwards 

 inferiorly. Its border slopes rapidly downwards and forwards for half its extent, 

 and then becomes semicircularly curved into the lower border without any 

 interruption. The inferior border is long, very feebly curved in front, but 

 straight in its posterior half, which ascends rapidly towards its termination and 

 joins the posterior border at a bluntly rounded angle. The posterior border is 

 straight, obliquely truncate from above downw r ards and backwards, making an 

 obscurely marked obtuse angle with the hinge-line. The hinge-line is arched in 

 front, but its posterior part, slightly produced, is straight. The umbones are 

 gibbose, incurved, twisted forwards, contiguous, raised above the hinge-line, and 

 situated in front of the middle point of the hinge-line. Proceeding obliquely 

 downwards and backwards to the inferior border, in front of the postero- 

 inferior angle, is a very well-marked, obtusely rounded ridge, behind which the 

 valves are very rapidly bent on themselves, so that the dorsal slope is very broad 

 and hollow transversely, and the extreme edges of the valve are compressed and 

 project backwards. The surface of the valves is convexly swollen above and 

 for half the dorso-ventral diameter, the lower half being gradually flattened and 

 expanded laterally. The greatest gibbosity of the valve is subumbonal. 



Interior. — The anterior adductor muscle-scar is very shallow, elongate, and 



