2U CRETACEOUS LAMELLIBRANCHIA. 



less distinct carina. When, however, a consideral)le nunil)er of specimens are 

 compared it is difficult to draw any line of separation between these forms and T. 

 Sanctse-Crucis. But since, in most cases, the original shape of the shell has been 

 more or less considerably modified l)y pressure, it is not easy to come to a definite 

 conclusion in this matter; moreover, in botli 7'. Sa)ictas-Grucis and the larger forms 

 there is clearly some variation in relative heii^'lit and length of the sliells, and in 

 the distinctness of the carina.^ 



JJistribidio)!. — Gault of Black Yen and Folkestone. Upper Greensand of 

 Blackdowu and Devizes. 



Thracia CARiNiFEifA {Soivevhi/), 182(). Plate XL, figs. 10 — 13. 



182(j. LuTEARiA r CAEiNiFERA, /. lie C. Soivefhy. Mill. C'oiuli., vol. vi, p. 66, pi. 



dxxxiv, fig. 2. 

 1832. • — —A. Passij. DescTipt. gi'ol. tie la Seiue-Iiift'r., 



Atliis, \>. 6, pi. xiii, figs. 6, 7. 

 1842. CoBiMYA CARINIFEKA. L. Ayassiz. Etudes crit. Moll. Foss., Myes, p. 264. 

 1845. Lyonsia carinifera, A. d'Orhiijni/. Pal. Franc. Terr. Crct., vol. iii. p. 385, 



pi. eeelxxiii, figs. 1, 2. 

 1850. — —A. cVOrblijiiy. Prodr. de Pal., vol. ii, p. 158. 



1852. — — A'. Ktier. Deukschr. d. k. Akad. d. Wisseiiscli. Wien, 



Math.-uat. CI., vol. iii. p. 311. 

 1854. LuTEARiA CARINIFERA, /. Morfis. Cat. Brit. Foss., ed. 2, p. 208 (r Thracia). 

 1865. Thracia carinifeka, F. J. Pictet aitd G. Campiche. Foss. Terr. Cret. Ste. 



Croix (Mati'r. Pal. Suisse, ser. 4), p. 121. 

 1870. — — F. Sfuliczht. Palaeout. ludiea, Cret. Fauua S. ludia, 



vol. iii, p. 72. 

 1893. Lyonsia carinifera, .4. Fc(7i?(7i. Stud. imGebietederbiJhru. Kreideformat. 



V. Prieseii. Si-liiclit., p. 97, fig. 115. 



Description. — Shell thin, oval, of moderate cunve.xity, slightly inequivalve and 

 inequilateral. Anterior margin rounded, jjassing gradually into the reguhirly 

 convex ventral margin. Posterior margin truncated, straight or slightly concave. 

 Postero-dorsal margin nearly straight. Umbones In'oad, incurved close together, 

 with a sharp carina passing to the postero-ventral angle and limiting a flattened 

 or slightly concave postero-dorsal area, whieh is tlivided in the middle l)y a shallow, 

 longitudinal fnri'ow. -lust in front of the carina is a broad, shallow, concave 

 depression. 



Ornamentation consists of broad, slightly raised, concentric folds Avhich are 

 more distinct on the anterior part than elsewhere. On the postero-dorsal area are 



^ A similar case of distortion and variation is i'urnislied liv Thracia semijilanata, Whiteaves, 

 ' Mesozoie Foss.' (' Geol. Surv. Canada'), vol. i (1884), p. 221, pi. xxix, fig. 5. 



