422 Mr. J. Lycett on Fossil Shells from the 



cated, flattened, with two obscure longitudinal ridges crossed by nu- 

 merous lines ; dorsal ridge nodulated, but little elevated ; dorsal costae 

 disunited from the carina, numerous, regular, narrow and curved. 

 With advance of growth the anterior transverse lines become indi- 

 stinct ; the dorsal ribs are broken in their anterior portions and dis- 

 placed, forming irregular nodules ; this change commenced when 

 twelve ribs had been perfected, but the Leckhampton specimens not 

 having that number, do not exhibit it. 



177. Triffonia Y.- costata ; semiorbicular, anterior side slightly 

 concave, with a single deep longitudinal groove and numerous trans- 

 verse prominent lines, prominent near the umbo, but becoming fine 

 and indistinct afterwards ; area lanceolate ; dorsal carina narrow, 

 smooth, but little elevated ; dorsal costse numerous, closely arranged, 

 angular and narrow, partially broken in the middle, and forming an 

 acute angle with their anterior portions ; at the middle they also be- 

 come slightly nodulated. 



178. Corburella, new genus. 



Gen. Char. Equivalve, inequilateral, transverse, thin, smooth ; um- 

 bones small, approximate, posterior side attenuated and slightly 

 gaping, anterior side more convex and rounded ; hinge with a small 

 depressed subconical cardinal tooth in each valve, and an extended 

 slightly thickened laminar plate forming a kind of anterior lateral 

 tooth or process ; muscular impressions faintly marked, scarcely 

 visible. 



This genus differs from Corbula in being equivalve, and in the cha- 

 racter of the hinge, the teeth are much smaller and not hollowed to 

 receive the ligament ; the substance of the test is thinner, and the 

 muscular impressions much more faintly marked. The Corbula cur- 

 tansata of Phillips is the type of this genus. 



179. Corbula imbricata ; shell suborbicular, small, slightly longer 

 than wide, imbricated by a few elevated concentric ridges rising over 

 each other ; lunule cordate, excavated ; umbones incurved : the largest 

 specimens attained the size of a small pea. 



1 77. Trigonia tuberculosa ; ovately trigonal, depressed, fornicated ; 

 anterior side flattened, transversely striated ; carina acute, elevated 

 and crenated ; ribs regular, curved and tuberculated, tubercles ele- 

 vated, obtuse, very closely arranged, their upper surfaces flattened. 



184. Cardium semicostatum -, ovately convex, rather longer than 

 wide ; umbones prominent, mesial, incurved, posterior side ribbed 

 longitudinally ; ribs smooth, rounded, closely arranged, occupying 

 about one-fourth of the suriface ; the remainder smooth in the adult 

 state, but yoiing individuals have very fine, closely arranged, concen- 

 tric strise. 



183*. Cypricardia siliqua ; transversely elongated ; umbones small, 

 anterior ; hinge-line very long, posterior side extended and attenuated, 

 ventral border straight, lines of growth few. 



187. Cardium Icevigatum ; suborbicular, smooth, transverse, mo- 

 derately convex ; umbones mesial, incurved, anterior side rounded, 

 posterior slightly truncated, ventral border rounded, lines of growth 

 few and obscure. 



