GERVILLTA. 



81 



sion from a small anterior portion, and l)y a lineai' depression from a very large 

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

 convex in older examples. This posterior portion lias 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 nut incurved. 



Ornamentation in the adult shell consists of mimerous growth-lamella}. On 

 the earlier part of the shell, and in young exam])l(>s, there are :i few rather strong, 

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



Fio. 10.— Gei-fiHia aJ.T/ormis (Soworby). Loft valve of specimen shown in Fig. 9. x J. 



Measurements 



Length of hinge 

 Height (oblique) 



(1) (2) 



97 . 00 

 122 .. 130 

 (1_3) Crackers, Atherfield. 



(3) 

 85 mm. 



116 „ 



Affimties.— This species presents some resemblance to G. nllnjidiensis 

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



Pictet and Campiclie regarded the form figured as G. aJxformh by d'Orbigny as 

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

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

 the Pmia-bed of Atherfield agree almost exactly with d'Orbigny's figure, and I 



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

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



