PECTEN. 167 
Prcoten (CHLAMYS) BRITANNICUS, sp. nov. Plate XXXI, figs. 1a, b, 2a, b. 
Description.—Shell thick, ovate, flattened, with even margins, slightly inequi- 
lateral, antero-dorsal margin a little longer than the postero-dorsal, apical angle 
about 9O°. 
Left valve with sixteen or more (sometimes probably thirty) strong, rounded, 
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 bifureate, 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 : : . ; ; ; : . 98°5 ,, 
From M. cor-anguinum zone, South Croydon. 
A ffinities—This species resembles P. Hspaillaci, @Orbigny,' from the Senonian 
of Dordogne and Charente-Inférieure, 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 imbricating upwards, instead of 
downwards. <A specimen of P. Mspaillact 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. 
P. fissicosta, Etheridge (p. 163), presents some resemblance to this species, but 
is easily separated by the divided and usually fewer mbs; also by the corrugated 
margin and the much less distinct concentric ridges, and by the occurrence of 
fine radial striz. 
1 «Pal. Frane. Terr. Crét.,’ vol. iii (1847), p. 614, pl. eceexxxix, figs. 1—4.; d’Orbigny, ‘ Prodr. de 
Pal.,’ vol. ii (1850), p. 251; F. J. Pictet and G. Campiche, ‘‘ Foss. Terr. Crét. Ste. Croix’’ (‘ Mater 
Pal. Suisse,” ser. 5, 1870), p. 215. 
2 «Petref. Germ.,’ vol. ii (1833), p. 53, pl. xcii, fig. 2; Geinitz, ‘Das Elbthalgeb. Sachsen,’ pt. ii 
(1872), p. 35, pl. x, figs. 8, 9. 
