264 CRETACEOUS LAMELLIBRANCHIA. 



A liroad, usually shallow sulcus extends from below the umbo to the ventral margin, 

 where it produces a slight sinuosity ; usually the sulcus starts at some distance from 

 the umbo, and may consequently be absent in small specimens. Ornamentation 

 consists of small, narrow, somewhat irregular concentric ribs, separated by broad, 

 shallow, concave interspaces. 



Afjlnities. — This species is related to I. concentricAis, Parkinson, but is dis- 

 tinguished by its subquadrate outline, by the length and height being nearly equal, 

 and by the presence of the sulcus. 



IxemarJiS. — All the specimens seen are internal casts of left valves. The 

 examples figured by Pictet and Roux and by Pictet and Campiche are also left 

 valves. 



Ti/2)p. — D'Orbigny's specimens came from the Albian of Novion, Clar, Geraudot, 

 and Saint Elorentin. 



Distribnfioji.^MammiUatiift bed of Copt Point, Folkestone. 



Inoceramus ang[,tcus, sp. nov. Plate XLV, figs. 8—10. Text-fig. 20. 



1822. Inoceramus, s])., G. Mantell. Foss. S. Downs, p. 96, pi. xix, fig. 20. 

 1859. — Crispii, T. Wiltshire. The Eed Chalk of England (Oeol. 



Assoc), p. 16, pi. i, fig. 4. 

 1875. — cONCENTEicxTS, A. J. Jukes-Browrie. Quart. Jouru. Geol. Soc, 



vol. xxxi, p. 299. 



Description. — Shell equiv^alve or nearly equivalve, very inequilateral. Anterior 

 part of shell convex, the convexity decreasing with age ; posterior part com- 

 pressed, flattened. Anterior slope of valves steep. Anterior margin moderately 

 convex ; ventral margin very convex ; posterior margin curved, and forming an 

 obtuse angle with the hinge-line. Length of hinge equal to rather more than a 

 third of the height of the shell. Unibones nearly terminal, with a small anterior 

 curvature. 



Ornamentation consists of strong, regular, concentric ribs, which have a sub- 

 symmetrical curvature, and become less distinct on the posterior flattened part of 

 the shell. The ribs have rounded summits, and are separated by broad rounded 

 furrows with symmetrical slopes. Some of the ribs bifurcate, some may be dis- 

 continuous, or new ribs may be intercalated. 



AffinUies. — The equal size of the valves, the greater convexity of the curve of 

 the ribs, the convex form of the anterior margin, and the outward slope of the 

 anterior part of the valves distinguish this species from I. neocomlensis, d'Orbigny 



(p. 2(;2). 



