POLLICIPES. 73 
pointed : the basal margin is short and straight; it forms, with the occludent margin, a 
right angle, and with the lower part of the tergo-lateral margin, an angle rather above a 
right angle. A quite narrow ridge, having perpendicular sides, only about twice as wide as 
the other ridges, runs in a slightly curved course from the apex to the baso-lateral angle ; 
at which angle the ridge, as it appears, must have formed a projection. The tergo-lateral 
portion of the valve, formed by the upturned zones of growth, is unusually broad, rather 
exceeding in width the rest of the valve; so that the ridge, running from the apex to the 
baso-lateral angle, divides the valve into two only slightly unequal portions, of which the 
tergo-lateral portion is the broadest. Internally, the upper part, above the pit for the 
adductor muscle, is along the middle, slightly prominent; on the tergal side, barely fur- 
rowed, and on the occludent side, slightly hollowed out. This species is allied to the 
three following species, and is, in some respects, intermediate between them and 
P. striatus. 
18. Potuicipes ricipus. ‘Tab. IV, fig. 7. 
PoLLiciPEs RIgIDUS. J. Sowerby. Trans. Geolog. Soc., 2d series, vol. iv, 1836, pl. xi, 
fig. 6* (carina et scuta). 
P. valvis transverse costatis: scutorum margine basali recto, cum margine occludente 
angulum recto majorem formante; costa angustissimd, parietali, ab apice ad angulum baso- 
lateralem decurrente : tergis, costé curvd, parietal, ad angulum basalem decurrente instructis ; 
apice basali in prominentiam parvam terminante, lateribus prominentie paralletis. 
Valves transversely ridged. Scwta with the basal margin straight, forming above a right 
angle with the occludent margin; a wall-sided, very narrow ridge runs from the apex to 
the baso-lateral angle. erga with a curved, wall-sided ridge running to the basal angle, 
which latter terminates in a little, parallel-sided projection. 
Gault: Folkstone and Maidstone, Kent. Eastweare Bay, Sussex. Mus. J. Sowerby, Bowerbank, 
Brit. Mus., J. Morris, Flower. 
General remarks. This species appears rather common. I have scuta, terga, and 
carinze, which I infer without hesitation belong to the same species, from the similarity of 
their peculiar surfaces, and from their having been found frequently at the same place and 
in the same formation. 
Description. All the valves have their surfaces conspicuously marked with sharp, nar- 
row, steep-sided prominent plaits parallel to the lines of growth ; each periodical zone of 
growth seems to have been completed by the formation of one of these projecting plaits ; 
the interspaces between the plaits, both on the scuta and carina, are either smooth, or more 
or less plainly fluted with fine longitudinal ridges; these apparently are in some instances 
k 
