258 CRETACEOUS LAMELLIBRAKCHIA. 



Description. — Shell large, rather thick, oval, elongate, inflated, the posterior 

 2iart compressed, very inequilateral. Anterior part very short, with rounded 

 margin, but its dorsal part only slightly curved. Ventral margin forming a 

 considerable curve. Posterior margin obliquely truncated, more or less rounded. 

 Postero-dorsal margin nearly straight. Umbones large, prominent, pointed, 

 iuroUed, 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 foi-ms the boundary' of a deeply excavated area in front of the umbones. 



Ornamentation consists of liroad, rounded, concentric ril)s on the dorsal part 

 of the valves except on the anterior excavated area. In passing vcntrally 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. 



Meastirements : 



(11 (2) (;i) 



Length . . 128 . 110 . 90 mm. 



Height . . 89 . 82 . 08 „ 



(1—3) Blackdown. 



Affinities. — This species resembles L. {Psilomya) .^iq-)erha (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. 



liemarks. — 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 Avhole of the valves. The radial ribs near the umbo are seen 

 in only a few specimens. 



Ttjpe. — From Blackdown, in the British Museum. 



Distrihntioii. — Upper Greensand (zone of Srlila'ulxicliid rustrata) of Blackdown. 



LioPiSTiiA, sp. Plate XLIII, fig. 5*^' — r. 



Description. — Shell oval, slightly inequilateral, inflated, posterior part com- 

 pressed, with a shallow furrow near the postero-dorsal margin, concave in front 

 of the umbones. Anterior and posterior uuxrgins rounded; ventral margin 

 considerably convex. Umbones prominent, curved inwards and forwards. 



Ornamentation consists of numerous (about twenty-nine), narrow radial ribs 

 l)eai'ing small tubercles, and separated l)y 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. 



Reinarldi. — The only exanq)le of this species which has been seen does not 

 1 ' PalseoTit. Indica, Cret. Fauna S. India ' (1870), p. 4.S, pi. iii, figs. 2 — 4. 



