62 CRETACEOUS LAMELLIBRANCHIA. 



Cucull^a nana, Leymerie, 1842. Plate XIII, figs. 1 a — e, 2, 3. 



1842. Ctjcull-Ea nana, A. Leymerie. Mem. Soc. Geol. de France, vol. v, p. 7, 



pi. ix, figs. 1 a — d. 

 1844. Aeca nana, A. d'Orbigny. Pal. Franc. Terr. Cret., vol. iii, p. 210, 



pi. cccxi, figs. 8 — 12. 

 1850. — — — Prodr. de Pal., vol. ii, p. 138. 



18(>6. — — F. J. Pictet and G. Campiche. Foss. du Terr. Cret. de Ste. 



Croix (Mater. Pal. Suisse, ser. 4), pt. 3, p. 472. 



Description. — Shell rhomboidal, inflated, inequilateral. Anterior margin 

 forming about a right angle with the dorsal, and curving ventrally. Ventral 

 margin nearly parallel to the dorsal, slightly curved. Posterior margin slightly 

 convex or sinuous, a little oblique. Umbones of moderate size, curving forwards, 

 more or less well separated, placed near the anterior third, with a carina extending 

 to the postero-ventral angle ; postero-dorsal area concave, but slightly raised near 

 the middle. Hinge-area long, broadest just in front of the umbones ; inner part 

 raised in front, with a parallel-sided groove and ridge on each side ; ligament- 

 grooves numerous. Ornamentation consists of numerous slightly elevated 

 concentric ribs, crossed by narrower radial ribs ; at the anterior end of the shell 

 some of the radial ribs at intervals are more elevated and serrate. Hinge not seen. 



Measurements : 



(i) 

 Length . . . 15 



Height .... 11 

 Thickness . . . 11*5 



Affinities. — This species is closely allied to Gucullsea subnana, Pictet and Roux, 

 but appears to differ in the form of the area, which is widest just in front of the 

 umbones, and tapers much more rapidly anteriorly than posteriorly. 



Remarks. — In many English specimens the area is wider and the umbones 

 more distant than in the example figured by d'Orbigny ; but others agree exactly 

 with his figure, and a series of specimens shows a gradual transition from forms 

 with a comparatively narrow area to those with a wide area. 



Types. — Specimens, but probably not those figured, are in the d'Orbigny 

 collection. 



Distribution. — Gault (zones ii, vii, viii, ix, and x) of Folkestone. Cambridge 

 Greensand (derived). 



12-5 



12 



9*5 mm. 



9 



9 



675 „ 



8 



9 



7 



