PR0T0SCH1Z0DUS OBLIQUUS. 235 



am able to figure the hinge of a left valve from a Belgian example of this species, 

 PL XVII, fig. 15 a. None of the British examples have any indication of this 

 structure preserved. 



This species is distinguished from P. axiniformis by the absence of the acute 

 angulated oblique ridge, and by the less regularly triangular and more obliquely 

 transverse shape of the shell. The inferior border is less curved, and the 

 postero-inferior angle less acute, while the anterior end is comparatively much 

 larger, and the umbones less pointed. 



Photoschizodus obliquus, M'Coy, sp., 1844. Plate XVIII, fig. 10; Plate XIX, 



figs. 7—9. 



Axinus obliquus, M'Coy, 1844. Synops. Carb. Foss. Ireland, p. 64, pi. viii, 



fig. 29. 

 — ? — Morris, 1854. Cat. Brit. Foss., edit. 2, p. 189. 



Non Myophoeia obliqua, M'Coy, 1855. Brit. Pal. Foss., p. 496. 



Scuizodus? obliquus, Bigsby, 1875. Thesaurus Devonico-Carboniferus, p. 311. 

 — — Etheridge, 1888. Brit. Foss., vol. i, Palaeoz., p. 290. 



Specific Characters. — Shell inequilateral, obliquely subtriangularly ovate, only 

 moderately convex. The anterior end forms barely one third of the valve trans- 

 versely, but is very deep from above downwards ; its border is almost semicircular, 

 so that the most anterior point is about the centre, and passes with regular 

 curvature into the inferior border, which is strongly convex downwards, but 

 forms the segment of a larger circle than the anterior border. The posterior 

 border is produced downwards and backwards ; very obliquely truncate and 

 almost straight, joining the inferior border at a well-marked, more or less blunted 

 angle. The hinge-line is very arcuate, at the shortest margin of the shell ; and the 

 posterior-superior angle is obtuse and fairly well marked. The umbones are 

 situated at the junction of the anterior and middle thirds of the shell, projecting 

 markedly upwards, moderately convex, small, incurved, and twisted forwards, 

 contiguous, only slightly elevated above the hinge-line ; excavated in front, so 

 that they appear to stand up high above the anterior part of the shell. Passing 

 downwards and backwards from the umbo close to the posterior margin is a 

 strong angular oblique ridge, which separates a compressed and very much 

 narrowed dorsal slope from the rest of the valve, which is regularly curved from 

 above downwards and before backwards, the greatest convexity being about the 

 centre of the valve. The ligament was external, small, and short. 



Interior. — The muscle-scars are normal in position ; pallial line entire. 



Exterior. — The surface is almost smooth, but in front there are indications of 

 regular concentric striations. 



