POLLICIPES. 73 



pointed : the basal margin is sliort 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 shghtly 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, shghtly 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. PoLLiciPEs RiGiDUS. Tab. 1\, fig. 7. 



PoLLiciPEs RIGIDUS. J. Sowerby. Trans. Geolog. Soc, 2d series, vol. iv, 1836, pi. xi, 

 fig. C* (carina et scuta). 



P. valvis transverse costatis : scutorum margine basali redo, cum margine occludente 

 angidum recto majorem formante ; casta angustissimd, parietali, ah apice ad angulum baso- 

 lateralem decurrente : tergis, costd curvd, parietali, ad angulum hasalem decurrente instructis; 

 apice basali inprominentiam parvam terminante, lateribus prominentia parallelis. 



Valves transversely ridged. Scuta 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. Terga 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 

 caringe, 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. 



Bescripition. All the valves have their surfaces conspicuously marked with sharp, nar- 

 roAv, 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 



