PECTEN. 167 



Pecten (CriLAMYs) BBiTANNicus, sp. nov. Plate XXXI, figs. 1 a, h, 2 a, h. 



Deserlptiou. — Shell thick, ovate, flattened, with even margins, slightly inequi- 

 lateral, antero-dorsal margin a little longer than the postero-dorsal, apical angle 

 abont 90^ 



Left valve with sixteen or more (sometimes probably thirty) strong, romided, 

 radial ribs, separated by deep grooves, which are frequently as broad as or 

 broader than the ribs. The ribs merge into the smooth margin of the shell ; 

 they do not bifurcate, and only rarely is a new rib introduced between two 

 others. Both ribs and grooves are marked by concentric, linear ridges, which are 

 more distinct in the grooves than on the ribs, and are placed close together at 

 regular intervals ; the ridges imbricate upwards. Anterior ear moderately large, 

 with the outer angle nearly rectangular, and one or two radial ribs. Posterior 

 ear not seen. 



Right valve not seen. 



Measurements : 



Length ........ 26"0 mm. 



Height 28-5 „ 



From M. cor-ancjuinmn zone, South Croydon. 



Affinities.' — This species resembles P. EspaUlaci, d'Orljigny,^ from the Senonian 

 of Dordogne and Charente-Inferieure, but it differs from the French form in 

 possessing fewer ribs, with a stouter shell and thick margin (instead of a sharp and 

 corrugated edge) ; also in the concentric ridges imljricating upwards, instead of 

 downwards. A specimen of P. EspaUlaci from the Dordonian has been sent me 

 by M. A. de Grossouvre, and its concentric ridges are much better developed on the 

 ribs, and in places are more scale-like, than in our species. 



P. decemcostatus, Goldfuss,^ possesses fewer ribs and is apparently without the 

 fine concentric ridges. 



F. fissicosta, Etheridge (p. 163), presents some resemblance to this species, but 

 is easily separated by the divided and usually fewer ribs ; also by the corrugated 

 margin and the much less distinct concentric ridges, and by the occurrence of 

 fine radial strife. 



1 'Pal. rran9. Terr. Cret.,' vol. iii (1847), p. 614, pi. ccccxxxix, figs. 1 — 4; d'Orblgny, ' Prodr. de 

 Pal.,' vol. ii (1850), p. 251 ; F. J. Pictet and G. Campiche, " Foss. Terr. Cn't. Ste. Croix" (' Mater 

 Pal. Suisse," ser. 5, 1870), p. 215. 



2 'Petref. Germ.,' vol. ii (1833), p. 53, pi. xeii, fig. 2 ; Geinitz, ' Das Elbthalgeb. Sachson,' pt. ii 

 (1872), p. 35, pi. X, figs. 8, 9. 



