DISCINA. 197 



Lingula, although the calcareous element is enormously greater in the last-named genus. The 

 chemical composition of the shell of Discina has been stated by Mr. S. Cloez to be 

 similar to that of Lingula, of which an analysis will be found further on. 



Discina appears to have existed during almost the entire series of Palaeozoic and 

 Mesozoic periods up to the present day, and it is probable that the animal was not at 

 any period the inhabitant of very deep water, for all the recent species of Lingula and 

 Discina, or those species with a horny shell, have prevailed in the littoral zone, and do not 

 appear to have descended deeper than about eighteen fathoms. The reader is referred for 

 more ample details to Prof. Suess's excellent ' Memoir on the Habitat and Distribu- 

 tion of the Recent and Fossil Brachiopoda ' recently published in Vienna. 



One or two British species only have been hitherto found in the Carboniferous 

 rocks. 



Discina nitida, Phillips. PI. XLVIII, figs. 18—25. 



Okbicula nitida, Phillips. Geol. of York., vol. ii, p. 221, pi. ix, figs. 10 — 13, 

 183G. 

 — cincta, Portlock. Report of the Geol. of Londonderry, &c, pi. xxxii, figs. 

 15, 16, 1843. 

 Discina bulla, M'Coy. British Palaeozoic Fossils, pi. iii d, fig. 32, 1855. 

 — nitida, Dav. Mon. of Scottish Carh. Brach., pi. v, figs. 22—29, 18C0. 



Spec. Char. Shell marginally circular or elongated oval, the posterior portion being 

 lather narrower than the anterior one. The larger or free valve is conoidal or limpet- 

 like, and more or less elevated, the pointed apex being situated at variable distances 

 between the centre and the posterior margin, but it is not always the most elevated portion 

 of the valve. The surface is covered with numerous small, irregular, concentric wrinkles 

 or striae. The smaller or lower valve is somewhat flattened or slightly concave towards 

 its anterior margin, with an oval-shaped foramen, surrounded by an elevated, convex 

 margin, which extends from near the centre of the valve to a variable distance from the 

 posterior edge. This valve is likewise ornamented with numerous small, irregular, con- 

 centric ridges or wrinkles, with small, flattened interspaces. No interiors have been 

 hitherto obtained. Dimensions variable ; three examples have measured — 



Length 10^, width 10, depth 6 lines. 

 7 7i ^ 



7 ^i A, 



Obs. After a lengthened examination of Discina cincta, as well as of Discina bidlu, 

 I could perceive no valid grounds for separating these two so-termed species from 

 D. nitida ; and any one possessing a sufficiently numerous series of specimens of the last- 

 named form would, I think, soon perceive that Phillips's shell presented every degree of 

 elevation — from that of an almost depressed shell to that extreme " inflated, bubble-like 



