(5) 



(6) 



(7) 



(8) 



(9) 



74 



66 



41 



54 



41mm 



152 



149 



89 



110 



80 „ 



76 CRETACEOUS LAMELLTBRANCHIA. 



Measurements : 



(1) (2) (3) (4) 



Hinge-line . . . . 92 86 86 83 

 Umbo to posterior extremity 182 200 182 163 

 (1 — 7) Crackers, Atherfield. 

 (8, 9) Greensand, Blackdown. 



Affinities. — GerviJUa sublanceolata is closely allied to G. cmceps, Deshayes, 1 of 

 which the types are from the Neocomian of Aube. The English specimens have 

 usually been referred to the latter species, but Pictet and Campiche regarded them 

 as distinct. 



The characters which separate the two species are (1) the antero-ventral margin 

 is concave in G. sublanceolata, whereas in G. anceps it is slightly convex or almost 

 straight ; (2) the posterior margin of the posterior wing-like part is convex, or in 

 some cases nearly straight, and the growth-lines on this part of the shell are convex, 

 whilst in G. anceps the corresponding margin and growth-lines are concave, and the 

 wing-like part is more distinct; (3) the line of greatest convexity — extending from 

 the umbo posteriorly — is near the middle of the valve in G. sublanceolata, but near 

 the antero-ventral margin in G. anceps ; (4) it is possible that G. sublanceolata is less 

 inequivalve than G. anceps? but at present this point cannot be proved, since only 

 a few specimens of the latter species showing both valves have been found. All the 

 examples known of G. anceps appear to be larger and to have thicker shells than 

 G. sublanceolata. 



Pictet and Campiche thought that G. anceps could be distinguished by the second 

 and third ligament pits being close together, whereas in G. sublanceolata the pits 

 are nearly equidistant. An examination of specimens of the former shows that the 

 position of the second and third pits, shown in d'Orbigny's figure, is an individual 

 variation, 3 and is not usually found. Pictet and Campiche mention as another 

 distinction the sharp line of separation between the posterior wing and the rest of 

 the valve in G. anceps ; although this feature is shown in d'Orbigny's figure it is not 

 evident in the specimens. 



G. sublanceolata differs from G. cosnenis, de Loriol, 4 in the rapid tapering of 

 the shell towards the posterior extremity and in the less extensive development of 

 the posterior wing-like part. 



Remarks. — Examples of this species from Atherfield were described and figured 

 as Gervillia alpina, Pictet and Roux, by Pictet and Renevier and by Pictet and 



1 ' Mem Soc. geol. de France,' vol. v (1842), p. 9, pi. x, fig. 3 ; D'Orbigny, ' Terr. Cret.,' vol. iii 

 (1846), p. 482, pi. cccxciv; Pictet and Campiche, 'Terr. Cret. Ste. Croix' (1869), p. 82, pi. civ, fig. 5. 



2 See Deshayes' fig. 3c. 



3 The probability of this has been mentioned by E. G. Skeat and V. Madseu, ' Jur. Neoc and 

 Gaidt Boulders in Denmark ' (' Danmarks geol. Undersog.,' vol. ii, No. 8, 1898), p. 163. 



4 ' Ganlt de Cosne' (1882), p. 83, pi. ix, figs. 21, 22. 



