SCHIZODUS PENTLANDICUS. 217 



Coal-measures of Conlbrookdale. 8. carbonarius seems limited to the Coal- 

 measures of Coalbrookdale and South Wales, while 8. antiquus characterises the 

 marine conditions of the Upper Carboniferous division of British rocks, taking 

 the top of the Carboniferous Limestone in Derbyshire, the top of the Yoredale 

 series of Wensleydale, and the top of the Carboniferous Limestone series of 

 Scotland as the upper limit of the lower division. The shaley nature of the 

 deposits in which the species of Schizodus occur probably points to their 

 habitat having been near the mouth of a large river, and possibly at no great 

 depth, but I am not clear as to whether they burrowed in the soft muddy ooze 

 or not. 



Schizodus Pbntlandicus, Ehiiid, sp., 1838. Plate XVI, figs. 2 — 9. 



Axinus Pentlandicus, Rhind, 1838. Age of the Earth, p. 1G7, pi. ii, figs, a, b. 

 Pachyodon pyramidatus, Capt. T. Brown, 1843. Ann. Mag. Nat. Hist., vol. xii, 



p. 396, pi. xvi*, fig. 9. 

 Unio ptramidatus, Capt. T. Brown, 1819. Fuss. Conch., p. 179, pi. lxxiii, fig. 19. 

 Myophoria carbonaria, M'Coy, 1855. Brit. Pal. Poss., p. 495. 

 Schizodus, sp. (Salter), Bev. T. Brown, 1861. Trans. Eoy. Soc. Edin., vol. xxii, 



p. 392, figs. 1—3. 



— Salteri, B. Ethcridge, jun , 1875. Ann. Mag. Nat. Hist., ser. 4, 



vol. xv, p. 431, pi. xx, figs. 6 — 9. 



— — — 1878. Quart. Journ. Geol. Soc, vol. 



xxxiv, p. 16. 



— — Bigsly, 1878. Thesaurus Devonico-carb., p. 311. 



Btheridge, 1888. Brit. Poss., vol. i, Pateoz., p. 290. 



Specific Characters. — Shell inequilateral, subquadrate, convexly swollen, espe- 

 cially in the umbonal region. The anterior margin is rounded, and passes with 

 a semicircular curve into the inferior border, which is gently convex downwards. 

 The posterior margin is a little shorter than the anterior, being narrowed by the 

 convergence of the upper and lower margins ; it is obliquely truncate from 

 above downwards and backwards, making bluntly rounded, slightly obtuse 

 angles Avith the hinge-line and ventral margin respectively. The upper border 

 consists of two portions, which slope rapidly upwards from each extremity to meet 

 in the region of the umbo at an acute angle. 



The umbones are large, tumid, situated in the anterior third of the upper 

 border, pointed, incurved, contiguous, and pointed very slightly forwards. The 

 valves are very convexly swollen in the umbonal region, and curve rapidly down- 

 wards to the ventral margin. In front the umbonal swelling is only marked off 

 from the rest of the valve at the anterior-superior angle; the convexity of the 

 anterior portion of the valve is very steep. Posteriorly, an obtusely rounded 



28 



