258 CRETACKOUS LAMELLIBRANCHIA. 
Description.—Shell large, rather thick, oval, elongate, inflated, the posterior 
part compressed, very inequilateral. Anterior part very short, with rounded 
margin, but its dorsal part only shghtly curved. Ventral margin forming a 
considerable curve. Posterior margin obliquely truncated, more or less rounded. 
Postero-dorsal margin nearly straight. Umbones large, prominent, pointed, 
inrolled, and with a more or less considerable forward curvature. A rounded 
carina extends from the front of the umbones to the middle of the anterior margin 
and forms the boundary of a deeply excavated area in front of the umbones. 
Ornamentation consists of broad, rounded, concentric ribs on the dorsal part 
of the valves except on the anterior excavated area. In passing ventrally these 
ribs become less prominent and are soon represented by concentric lines. Rather 
widely separated radial rows of minute tubercles are present and leave small pits 
when broken off. Near the umbo small radial ribs are present. 
Measurements : 
(1) (2) (3) 
Length f . 128 ; 110 : 90 min. 
Height : : 89 , 82 3 hol tas 
(1—3) Blackdown. 
Affinities —This species resembles L. (Psilomya) superba (Stoliczka!), but 
differs in outline, in the concentric ribs being confined to the dorsal part of the 
shell, and in the less inflated form of the valves. 
Remarks.—In the small examples of this species the shell is relatively shorter 
and more nearly orbicular in outline and the concentric ribs cover the whole 
or nearly the whole of the valves. The radial ribs near the umbo are seen 
in only a few specimens. 
Type.—From Blackdown, in the British Museum. 
Distribution.—Upper Greensand (zone of Schlanbachia rostrata) of Blackdown. 
LiopistHa, sp. Plate XLITI, fig. 5a—e. 
Description.—Shell oval, shghtly mequilateral, inflated, posterior part com- 
pressed, with a shallow furrow near the postero-dorsal margin, concave in front 
of the umbones. Anterior and posterior margins rounded; ventral margin 
considerably convex. Umbones prominent, curved inwards and forwards. 
Ornamentation consists of numerous (about twenty-nine), narrow radial ribs 
bearing small tubercles, and separated by concave interspaces of greater breath 
than the ribs. On the postero-dorsal part of the shell ribs are absent. The 
ribs are crossed by faintly marked concentric rings. 
Remarks.—Vhe only example of this species which has been seen does not 
1 «Paleout. Indica, Cret. Fauna 8. India’ (1870), p, 48, pl, ii, figs. 2—4. 
