62 CRETACEOUS LAMELLIBRANCHIA. 
CucutLmA NANA, Leymerie, 1842. Plate XIII, figs. 1 a—e, 2, 3. 
1842. CuctLnma nana, A, Leymerie. Mém. Soe. Géol. de France, vol. v, p. 7, 
pl. ix, figs. 1 a—d. 
1844, Arca nana, 4. d’Orbigny. Pal. Franc. Terr. Crét., vol. ii, p. 210, 
pl. ecexi, figs. S—12. 
1850. — — _ Prodr. de Pal., vol. ii, p. 188. 
1866. — — FJ. Pictet and G. Campiche. Foss. du Terr. Crét. de Ste. 
Croix (Matér. 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 : 
(1) (2) (3) (4) 
Length : : ; 15 12°5 12 975 mm. 
Height . 3 : ; il 9 9 G27Dy lee 
Thickness. 5 , 11°5 8 9 G - 
Affinities.—This species is closely allied to Cucullza 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). 
