24G CARBONIFEROUS LAMELLIBRANCHIATA. 



depressed and extended hinge-line, and the small inconspicuous umbones, which 

 have no sign of an anterior fold, are characteristic. This condition of the umbo 

 at once separates P. magnus from P. sequilateralis, in which the anterior fold is 

 very well marked. 



Mr. J. Wright's specimen, fig. 23, PL XIX, is a cast from Little Island, 

 co. Cork, but unfortunately details are not well shown, and it is imperfect at the 

 posterior end ; and this is also, to a smaller extent, the case with the other 

 specimen, fig. 22, PI. XIX; but the general contour can be well made out from 

 the two examples, for the latter specimen is only imperfect below, while the 

 former is so above. The absence of any oblique ridge is also very marked — a 

 feature present in most of the species of this genus. In the absence of this 

 character P. magnus agrees with P. impressus, but the umbones of the latter 

 species are much better marked off from the shell and raised above the hinge-line, 

 and not median. The external ligament is well preserved in the Survey specimen, 

 and is very small and short. 



De Koninck gives three figures of a single specimen of this species, which, he 

 states, is very rare, and occurs in the Lower Carboniferous of Tournai. His 

 specimen shows the hinge, which is normal. The specimen is less perfect even 

 than those I figure, and this fact accounts for the slight difference in his 

 description of the contour of the valve and mine. He remarks on the feebleness 

 of the diagonal ridge, so common to other species of the genus. 



Protoschizodus ^quilateralis, M'Coy, sp., 1844. PI. XX, figs. 5, 7, 10 — 12. 



Leptodomus fragilis, M'Coy, 1844. Synops. Carb. Foss. Ireland, p. 07, pi. x, 



fig. 11. 

 Dolabra ^quilatebalis, M'Coi/, 1844. Ibid., p. 65, pi. xi, fig. 14. 



— — Morris, 1854. Cat. Brit. Foss., edit. 2, p. 202. 



Leptodomus feagilis, Morris, 1854. Ibid., p. 206. 



— — (pars.), B. Etheridge, jun., 1876. Ann. Mag. Nat. Hist., 



ser. 4, vol. xviii, p. 101, pi. iv, figs. 6, 7. 

 Protoschizodus insigkis, de Koninck, 1885. Add. Mus. Boy. Hist. Nat. Beige, 



vol. xi, p. 128, pi. xxii, fig. 10. 

 Dolabra jjquilateealis, Etheridge, 1888. Brit. Foss., vol. i, Palaeoz., p. 283. 



Specific Characters. — Shell of moderate size, subquadrate, oblique, regularly 

 gibbose, except at the postero-superior angle, which is compressed, produced, and 

 subalate. The anterior end is much shorter than the posterior, regularly convex, 

 and has its border semicircularly curved, which passes into the ventral border 

 without a break. The latter is convex, the arc of a larger circle than that of the 

 anterior border. The posterior border is almost straight, truncate, and slightly 



