202 CRETACEOITS LAMELLTBRANCHTA. 



JJrsrripfinu. — Shell siuall, inflated, oval, a little higher than long, slightly 

 inecjuilateral. Anterior margin rounded, passing gradually into the convex 

 ventral margin. Posterior margin sul^truncate, higher than the anterior margin. 

 UmVwnes prominent, with a faint carina extending to the postero- ventral extremity 

 and forming tlu^ limit of the flattened postero-dorsal area. Margins of valves 

 serrate. 



Ornamentation consists of numerous small, slightly-raised radial ribs separated 

 by narrow grooves ; anteriorly the ribs become gradually smaller and are absent 

 or indistinct near the antei'o-dorsal margin ; on the postero-dorsal area the ribs 

 are stronger and the grooves broader than elsewhere, and the anterior margins of 

 these ribs are sometimes serrate. In Avell-preserved specimens faint concentric 

 linear ridges are seen. 



Meafnimnrvfa : 



(1) (2) (3) 



Length .17 . 15-5 . 14-2 . 

 Height .18 . 16-5 . 15 



(1—5) Crackers. AthertieUl. 



Affiuifirs.—iiec G. GuttaUmnm (p. 203). 



In form C. Ibbetsoni resembles 0. Banliniannm, d'Orbigny,^ but the latter is 

 distinguished by its broader grooves which bear pointed projections.- 



In the specimens figured by Pictet and Renevier from the Aptian of the Perte 

 du Rhone the umbones are more prominent than in English examples of 

 0. Ihhdsoni, but Pictet and Campiche, who were able to compare examples from 

 Atherfield with those obtained from the Perte da Rhone, felt no doubt as to 

 their specific identity. 



Ti/pe. — From Atherfield, iu the Museum of the Geological Society. 



Distribution. — Lower Grreensand (Crackers) of Atherfield. 



' -Pill. Frauv- Terr. Cn't.,' vol. iii (1844j, p. 25. pi. cc-xlii, fi.i,'s. 7—11. 



' C. Bauliniannm is rocorJed by Morris from the Lower Greensaml nt' t ln' Isle of Wight, aud by 

 Topley from the Atherlield Beds of Peasmarsh and Shalfonl. Specimens from the Atherfiehl Clay 

 wei-e referred to C. suhhUlaimm, Leymerie, by Forbes, but that identilication was regarded as doubtful 

 by Pictet and Campiche ; the form of the shell cannot be made out satisfactorily, but the ornamenta- 

 tion resembles that of G. Ibbetsoni. Au internal cast from the Lower Grreensand of Upware was 

 referred with doubt to C. unhhillaitnin by W. Keeping (' Foss. Neoc. Upware and Brickhill,' 1883, 

 p. 119) ; the specimen is now in the Sedgwick Museum, Cambridge, and seems to me insufficient 

 for determination. A specimen with radial ribs, from the Lower G-reensand of Maidstone, was named 

 CartJium Bemtedi by Forbes, but was too imi)erfect for figuring ; the type is in the Museum of the 

 Geological Society (No. 2124) ; no other specimen has been seen. Forbes, 'Quart. Jouru. Geol. Soc.,' 

 vol. i (184.5), p. 244 ; Pictet and Canii)ichc, ' Foss. Terr. Cn't. Ste. Croix ' (' Mater. I'al. Suisse,' ser 4, 

 186G). p. 267. 



