PECTEN. 187 
1841. Prcren asper, Ff’. A. Rimer. Die Verstein. d. nord-deutsch. Kreidegeb., 
p. 53. 
1846. ~- —  H. B. Geinitz. Grundriss der Verstein., p. 469. 
— — — A. FH. Reuss. Die Verstein. der bihm. Kreideformat., pt. 2, 
p 30, pl. xl, fig. 1. 
1847. - — A. @Orbigny. Pal. Franc. Terr. Crét., vol. iii, p. 599, 
pl. cecexxxiv, figs. 1—6. 
L850. — — H. B. Geinitz. Das Quadersandst. oder Kreidegeb. in 
Deutschland, p. 184. 
A. d’Orbigny. Prodr. de Pal., vol. ii, p. 168. 
1852. — — £. Kner. Denkschr. d. k. Akad. d. Wissensch., Math.-nat. 
CL, vol. ii, p. 317, pl. xvii, fig. 6. 
1854. — — J.Morris. Cat. Brit. Foss., ed. 2, p. 175. 
1855. —- —  G. Cotteau. Moll. Foss. de ’Yonne, p. 116. 
1863. -- — A. Kunth. Zeitschr. d. deutsch. geol. Gesellsch., vol. xv, 
p. 724. 
1870. — — F.J. Pictet and G. Campiche. Foss. Terr. Crét. Ste. Croix 
(Matcér. Pal. Suisse, ser. 5), pp. 208, 213. 
1871. —  (Cuuamys) asper, I. Stoliczka. Paleont. Indica, Cret. Fauna S. 
India, vol. iu, p. 428. 
1872. —  asper, H. B. Geinitz. Das Elbthalgeb. in Sachsen (Paleonto- 
graphica, vol. xx, pt. 1), p. 198. 
1876. — — H. Deicke. Die Tourtia von Miilheim a. d. Ruhr, p. 26. 
1878. Curamys asper, LY. Bayle. Explic. de la Carte géol. de France, vol. iv, 
pt. 1 (Atlas), pl. exxu, fig. 1. 
1895. Prcren asper, R. Michael.  Zeitschr. d. deutsch. geol. Gesellsch., vol. 
xlv, p. 234. 
Non 1830, — — Hart (fide @Orbigny). 
— 1852. = — IL. Reeve. Couch. Icon., vol. viii, pl. ii, fig. 10. 
Description—Shell usually rather large, nearly circular, equilateral, almost 
equivalve ; antero- and postero-dorsal margins of equal length, straight or slightly 
concave. Valves moderately convex—right valve a little more convex than the 
left. Length equal to, or shghtly greater than, height. Umbones sharp. Apical 
angle about 105°. Kars moderately large, unequal. 
Surface of both valves ornamented with about seventeen main ribs, which are 
angular and elevated ; near the anterior and posterior borders they are sometimes 
rather smaller than elsewhere ; on the ribs are numerous sharp, hollow, ventrally 
directed spines. Near the umbo only these main ribs occur, but at a little dis- 
tance from it another similar but smaller rib, with similar spines, is introduced 
on each side of the main rib; and later other ribs usually appear successively 
—often one or two, but sometimes three or more on each side of the main rib. 
These lateral ribs are not always of equal size, and not always placed at equal 
distances from one another. The antero- and postero-dorsal margins of the valves 
