o — 



easurements : 





(i) 



(2) 



Length .17 



15*5 



Height . 18 



16-5 



(4) 



(5) 



14 



10'5 mm 



1 K> . 



11 „ 



202 CRETACEOUS LAMELLTBRANCHIA. 



Description. — Shell small, inflated, oval, a little higher than long, slightly 

 inequilateral. Anterior margin rounded, passing gradually into the convex 

 ventral margin. Posterior margin subtruncate, higher than the anterior margin. 

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

 and forming the 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 antero-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 well-preserved specimens faint concentric 

 linear ridges are seen. 



(3) 



14-2 

 15 



(1 — 5) Crackers, Atherfield. 



Affinities. — See G. Gottaldinuni (p. 203). 



In form G. Ibbetsoni resembles G. Raulinianum, d'Orbigny, 1 but the latter is 

 distinguished by its broader grooves which bear pointed projections. 2 



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 

 G. Ibbetsoni, bnt Pictet and Campiche, who were able to compare examples from 

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

 their specific identity. 



Type. — From Atherfield, in the Museum of the Geological Society. 



Distribution. — Lower Greensand (Crackers) of Atherfield. 



1 'Pal. Fran?. Terr. Crct.,' vol. iii (1844), p. 25, pi. ccxlii, figs. 7—11. 



2 C. Raulinianum is recorded by Morris from the Lower Greensand of the Isle of Wight, and by 

 Topley from the Atherfield Beds of Peasmarsh and Shalford. Specimens from the Atherfield Clay 

 were referred to C. subhillanum, Leymerie, by Forbes, but that identification 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 C. Ibbetsoni. An internal cast from the Lower Greensand of Upware was 

 referred with doubt to C. Bubhillanum 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 Greensand of Maidstone, was named 

 dunlin hi Benstedi by Forbes, but was too imperfect for figuring ; the type is in the Museum of the 

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

 vol. i (1845), p. 244 ; Pictet and Campiche, ' Foss. Terr. Crct. Ste. Croix ' (' Mater. Pal. Suisse,' ser 4, 

 1866), p. 267. 



