126 SYNOPSIS OF THE CHARACTERS OF THE 



much nearer to the O.Jiliaria of the superior beds, but has much coarser strias, and a larger hinge-line. Length 

 nine lines, width eight lines. 



OrTHIS (?) QTJADRATA. M'CoTJ. (PI. XX. fig. 18). 



Sp. C/i. — Shell quadrate, pyramidal; front straight, slightly emarginate; sides slightly concave ; cardi- 

 nal angles pointed ; cardinal area very large, fiat, triangular ; surface with about forty strong, radiating ridges, 

 alternately larger and smaller. 



This pretty little shell resembles the Leptcena minima of the Wenlock shale in outline and size, but the 

 striffi are coarser, and the cardinal area larger ; the present species is almost pyramidal, the height at the beak 

 being rather more than one-half the length. Only a single specimeu has occurred. Length one line and a 

 half, width two lines. 



Orthis radialts. Phil. sp. 



Sjiirifera radialis. Phil. Geol. York. 



Sp. Ch. — Semicircular, twice as wide as long; convex; surface with large, irregular, rounded, radiating 

 ridges, with numerous, intermediate, smaller ones, all crossed by numerous, prominent, imbricating laminai of 

 growth. 



This magnificent shell is in no danger of being confounded with any other, its characters are so bold and 

 striking. It is very rare. 



Orthis resupinata. Mart. sp. 



Con. Anom. resupinatus. Mart. Pet. Derb. — Terebratula resupinata. Sow. Min. Con. — Spirifera resupinata. Phil. 



Geol. York Orthis resupinata. Phil. Pal. Fos. 



Sp. Ch. — Transversely oval, length one-third less than the width; dorsal valve concave ; ventral valve 

 evenly convex, with a mesial depression ; surface with fine, radiating striae, which form lengthened, alternating 

 spines at irregular intervals. 



This species is liable to considerable variation, the striaj are often nearly plain, some are more convex than 

 others, and finally some adult specimens are scarcely an inch long, while others, apparently not full grown, are 

 nearly three times that size, just as we see in some living species of CyprcBa. Length one inch five lines, 

 width one inch ten lines, depth ten fines. 



Orthis semicircularis. Soiv. 



Orthis semicircularis? Sow. Sil. Syst. — Orthis semicircularis. Sow. Geol. Trans — Orthis semicircularis. Phil. Pal. Fos. 



Sp. Ch. — Truncato-orbicular, gibbous; radiating striae, fine, direct, increasing in number towards the mar- 

 gin ; cardinal teeth very short, thick, and very much diverging. 



I have followed Professor Phillips and Mr. Sowerby in calling this Devonian shell O. semicircularis, 

 although the Caradoc sandstone species appears to me to difier constantly in being wider, more convex, and 

 more coarsely striated. 



Orthis sulcata. M'Coy. (PI. XX. fig 6). 



Sp. Ch. — Semicircular, gibbous; ears flattened; surface with very coarse, rounded, frequently branched 

 striffi; hinge-line exceeding the width of the shell, furnished with ten strong, conical spines. 



This species is easily distinguished from the Leptcma volva, M'Coy, by its flattened ears, and coarser striae, 

 and from O. multidentata, M'Coy, by its greater gibbosity, coarser striaj, and less number of cardinal spines. 

 Length nine lines, width fourteen fines. 



Orthis (?) tenuistriata. Sow. 



Orthis tenuistriata. Sow. Geol. Trans. 

 I have referred to this species some imperfect fragments of a shell most nearly related to the Leptcma 

 Hardrensis, Phil, but with the strife somewhat finer. The specimens to which I allude are certainly not the 

 young of the O. crenistria, which has much wider and more unequal striae. 



