76 CRETACEOUS r.AMEETJBRANCHIA. 



Measurements : 



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



Hinge-line . . . . 92 86 86 83 

 Umbo to posterior extremity 182 200 182 163 



(1 — 7) Crackers, Atherfiekl. 

 (8, 9) Greensand, Blackdown. 



Affinities. — Gervillia snhlanceolata is closely allied to G. anceps, Deshayes,' of 

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

 usually been referred to the latter species, biit Pictet and Civmi)iclic regarded thcni 

 as distinct. 



The characters which separate the two species are (1) tlic antero-ventral mai'giu 

 is concave in G. suhhtnceolata, 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 tliis part of the shell are convex, 

 whilst in G. anceps tlie 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. suhlanceolata, but near 

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

 incquivalvo tliaii G. avccps,- but at present tliis point cannot l)e prove<l, since only 

 a few specimens of the latter species showing botli vnlvcs liavo ])een Found. All the 

 examples known of G. am-c/is appear to l)(^ larger ami to Imvc thicker shells than 

 G. t<ul)lauccolitla. 



I'ictetand Campiehe thought that (/. ((iiccps could be distinguished by the second 

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

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

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

 variation," and is not usually found. I'ictet and Cfimpiche mention as another 

 distinction the sharp line of se})aration 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. suhlanceolata differs from G. cosnenis, de Tjoriol,' 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 Atherfiekl were described and figured 

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



• 'Mc'm Soc. gcol. de France,' vol. v (1842), p. 9, pi. x, fig. 3; D'Orbigny, 'Terr. Cret.,' vol. iii 

 (1846), p. 482, pi. cccxciv; Pictet and Campiehe, 'Terr. Crut. Ste. Croix' (1869), p. 82, pi. civ, fig. 6. 



2 See Desbayes' fig. 3c. 



•'' The probaliility of this has licen mentioned hy E. G. Skeat and V. Madson, ' Jur. Ncoc. and 

 Ganlt Boulder.s in Denmark' (' Dauniarks geol. Undersog.,' vol. ii, No. 8, 1898), p. 163. 



* ' Gault de Cosne' (1882), p. 83, pi. ix, figs. 21, 22. 



