GERVILLIA. 79 



Description. — Shell small, thin, elongate, compressed, very oblique, angular 



anteriorly, truncated posteriorly. Left valve more convex than the right. Urn- 

 bones almost terminal. Ligament area narrow, with four or five pits, one of which 

 is under the umbo. Anterior part of the shell very small, compressed. Median 

 part flattened. Postero-dorsal part relatively large, compressed, wing-like. On 

 the left valve a rounded ridge extends from the umbo to the postero-ventral angle; 

 below this ridge the shell is bent sharply. 



Surface smooth, or ornamented with concentric lines. 

 Measurements : 



(i) (2) (3) 



Length of hinge . . . 15*5 . 15 . 13 mm. 



Umbo to postero-ventral angle 23 . 29 25 „ 



(1—3) Crackers, Atlierfiekl. 



Affinities. — G. recta, Meek and Hayden, 1 from the Fox Hill Beds of the Upper 

 Missouri, is closely allied to this species. The imperfectly known Gervillia Eeichi, 

 Romer, 2 resembles G. linguloides, but appears to have the posterior wing more 

 distinctly limited. 



In the form of its shell G. linguloides resembles some of the species of Pteria 

 which belong to the sub-genus Pseudoptera (see p. 63), but the presence of liga- 

 ment pits proves it to be a Gervillia. 



Remarks. — Pictet and Renevier showed that Avicula ephemera, Forbes, is only 

 an internal cast of Gervillia linguloides. 



This species occurs commonly in the Crackers of Atherfield and is gregarious. 



Types. — From the Crackers of Atherfield, in the Museum of the Geological 

 Society (Nos. 2040, 2054). The types of Avicula ephemera, also from Atherfield, 

 are in the same collection (Nos. 2051, 2052). 



Distribution. — Lower Greensand (Crackers) of Atherfield. Recorded by Topley 

 from the Atherfield Clay of Peasemarsh and Shalford. 



Gervillia aljeformis (Sowerby), 1819. Plate XI, figs. 9a-d, lOa-d, 11. Text- 

 figures 9-14. 



1811). Modiola? ai,.kfok.iiis, ./. Sowerby. Mill. Coiu'h., vol. iii, p. 93, pi. ccli. 

 1835. Pkrna alifokmis, Sowerby. Ibid., vol. vi, systematical index, p. 243. 

 1845. — AL.EFORMIS, E. Forbes. Quart. Journ. Geol. Soc, vol. i, p. 246, pi. iii, 

 %■ 2. 



1 ' Proc. Acad. Nat. Sci. Philad.' vol. xiii (1861), p. 441. F. B. Meek, ' Invert. Crct, and Tert. Fobs. 

 U. Missouri ' (1876), p. 60, pi. xxix, fig. 1 a, b. 



2 ' Die Verstehi. d. nord-deutsch. Kreidegeb.' (1841), p. 04, pi. viii, fig. 14 (named G. Cott;e on 

 pi. viii). 



