GBRVILLIA. 81 



sion from a small anterior portion, and by a linear depression from a very large 

 triangular posterior portion, which is flattened in small specimens but moderately 

 convex in older examples. This posterior portion has a wing-like projection in 

 young specimens, but in older forms its posterior border is nearly straight and 

 forms an obtuse angle with the hinge-line. 



Right valve similar to the left, but smaller, less convex, with the posterior 

 portion more flattened, and with the umbo not incurved. 



Ornamentation in the adult shell consists of numerous growth-lamellae. On 

 the earlier part of the shell, and in young examples, there are a few rather strong, 

 broad, rounded radial ribs with a few smaller ribs between. 



Pig. 10. — Gervillia alxformis (Sowerby). Left valve of specimen shown in Fig. 9. x i. 



Measurements : 



(1) (2) (3) 



Length of hinge 97 . 90 85 mm. 



Height (oblique) . 122 .130 . 116 „ 



(1—3) Crackers, Atherfield. 



Affinities. — This species presents some resemblance to G. allaudiensis 

 (Matheron l ) but is more inflated, less inequivalve, and less oblique. 



Pictet and Campiche regarded the form figured as G.ulxformis by d'Orbignyas 

 distinct from Sowerby's G. alxformis, and they believed that the former was limited 

 to the Neocomian whereas the latter occurs in the Aptian. Some specimens from 

 the Perna-hed of Atherfield agree almost exactly with d'Orbigny's figure, and 1 



1 ' Catal. Foss. des Bouches-du-Rhone ' (1842), p. 175, pi. xxvi, fig. 1 ; Pictet and Campiche, ' Terr. 

 Cret. Ste. Croix' (18G9), p. 81, pi. civ, fig. 1 



11 



