84 CRETACEOUS LAMELI.JBRANCHIA. 



1850. Pkkna eostrata, cVOrhifjnij. Ibid., p. 168. 

 1854. Gervillia — J. Morris. Cat. Brit. Foss., ed. 2, p. 168. 

 1871. Melina — F. Stoliczka. Palseout. Indica, Crct. Faumi S. India, 



vol. iii, p. 400. 

 ? 1895. AvicuLA cf. cenomanensis, E. Tiessen. Zeitschr. d. deutsch. geol. Gesellscb., 



vol. xlyii, p. 479. 



Description. — Shell rather small, of moderate convexitj'-, often very oblique, 

 triangular. Ventral and posterior margins rounded. Hinge-line long. 



Left valve more convex than the right, with the umbo moderately incurved. 

 The large, central, very convex portion is indistinctly separated from the large, 

 anterior, triangular, wing-like ear and from a narrow, flattened, obtusely triangular 

 posterior part. 



Right valve similar to the left but less convex and with the umbo only slightly 

 incurved, and with the anterior ear more distinctly limited. 



Surface of valves with narrow, regular growth-layers. 



Measurements : 



(1) (2) (3) 



Length of hinge . 21 . 20 . 20 . 



Height (oblique) . 25 . 28 . 24 . 



(1 — 5) Greeusand, Blackdowu. 



Affinities. — Avicula cenomanensis, d'Orbigny, from the Cenomauian of Le Mans, 

 appears to be identical with G. rostrata. In all the specimens of the latter which 

 I have seen, the terminal portion of the posterior wing is more or less imperfect, 

 but the growth-lines show that the posterior margin must have had the same form 

 as in d'Orbigny's fig. 11. 



Gervillia rostrata presents some resemblance to the young fonn,s of G. alieformis 

 (p. 79) but is more oblique and without radial ribs, also the central convex 

 portion is less sharply marked off from the lateral parts, and the anterior ear is 

 larger. 



G. rostrata is allied to G. tciiuicostala, Pictet and Campiche (see below), but the 

 right valve is less flattened, and the concentric ornamentation appears to be less 

 developed — this, however, may be due to difference of preservation, since some 

 of the Blackdowu specimens are neai'ly smooth whereas others show di.stinct 

 concentric ridges. 



Remarks. — This species is moderately common at Blackdowu but is usually 

 imperfectly preserved. An example from the Gault of Folkestone, recorded by 

 Price as Avicula cenomanensis, is probably referable to this species, but the greater 

 part of the shell has disappeared, leaving a mould of the right valve ; the specimen 

 is now in the Museum of Practical Geology (No. 1(324). 



The examples of G. rostrata show a considerable amount of variation in ul)li(|uity. 



