14 CRETACEOUS LAMELLIBRANCHIA. 



Affinities. — I have seen only a few examples of this form. They agree with 

 L. villersensis except in having a rather smaller apical angle. In this respect they 

 resemble L. Orbignyana, Matheron (see above), but they differ from that species in 

 possessing fewer and straighter ribs. I have not seen any undoubted example of 

 L. villersensis and am unable to state whether the apical angle is constantly larger 

 than in the English specimens. Pictet and Campiche say that it is about 95°, but 

 the specimen they figure possesses an apical angle of 90° only. In the English 

 specimens it is about 85°. 



This is the form which was referred by Sharpe 1 to L. consobrina, d'Orbigny, 

 but it possesses considerably fewer ribs than that species. 



Type. — L. villersensis is found in the Valanginian of Ste. Croix. 



Distribution. — Lower Greensand of Faringdon. 



Lima (Plagiostoma) semiornata, d'Orbigny, 1847. Plate III, figs. 14, 15, 16a, b. 



Plate IV, fig. 1. 



1847. Lima semiornata, A. d'Orbigny. Pal. Franc;. Terr. Oct., vol. iii, p. 555, 



pi. ccccxxii, figs. 1 — 3. 

 1850. d'Orbigny. Prodr. de Pal., vol. ii, p. 167. 



1867. — E. Gueranger. Album Palcout. de la Sartlie, p. 19, 



pi. xxiv, fig. 13. 



1870. F. J. Pictet and G. Campiche. Foss. Terr. Oct. Ste. 



Croix (Mater. Pal. Suisse, ser. 5), 

 p. 169. 



1871. Eadula (Plagiostoma) semiornata, F. StoliczJca. Palaeont. Indica, Cret. 



Fauna S. India, vol. iii, p. 414. 

 ? 1885. F. mtling. Die Fauna d. baltisch. 



Cenoman. (Palaeont. Abliandl., 

 vol. ii), p. 15, pi. ii, fig. 4. 



Description. — Shell compressed, rounded, height and length nearly equal. 

 Antero-dorsal border straight or slightly concave. Umbones close together. 

 Anterior area depressed, with a sharp edge. Apical angle 120°. 



Surface nearly smooth, shiny, with numerous, very fine, regular, concentric 

 linear ridges, and with radial punctate grooves near the umbo and near the 

 anterior and posterior margins. The grooves near the anterior margin are fewer 

 and more widely separated than those near the posterior margin. 



1 'Quart. Journ. Geol. Soc.,' vol. x (1853), p. 193 (suh-cunsuhrina, d'Orbigny, 'Prodr. de Palcont.,' 

 (1850), p. 167). 



