38 MOLLUSCA FROM THE CRAG. 



This, as yet, I have seen from the Red Crag only, and in that Formation it is by 

 no means abundant. 



Some varieties of P. opercularis seem to approach this so closely, that it is possible 

 it may be only a modification of that variable species, with more attenuated and 

 depressed valves than are commonly seen ; there are, however, some differences which 

 must, at least for the present, keep them separated, more especially as there is no 

 necessity for the imp osition of a new name. The variety linearis of that species, in the 

 form and arrangement of its sculpture, appears to approach the nearest to our shell. 



In P. gracilis both valves are much flattened, the right valve rather the more so 

 of the two ; the auricles appear comparatively less than in opercularis, and the rays 

 are arranged much in the same manner, but rather more numerous, varying from 

 twenty to twenty-six, generally tripartite or ranged in threes, the centre one the most 

 elevated and the most sharp, with an intermediate one between the three, so that 

 every fourth ray, as stated by Sowerby, ' Min. Conch.,' vol. iv, p. 129, is the most 

 prominent and the most conspicuous ; the same disposition of the rays may be 

 observed in opercularis, var. linearis, only in that shell, neither the valves nor the 

 rays are so much depressed. Our shell is ornamented with fine concentric striae, or 

 raised and subimbricated lines of growth, giving a roughness to the feeling, more than 

 to the eye, but the same sculpture is present in linearis. 



The most material differences are a greater flatness in the valves, a much 

 thinner shell, with a slight alteration in proportionate or comparative dimensions, and 

 smaller auricles ; this last may, perhaps, be looked upon as the most distinguishing 

 character, if they be really distinct, which more numerous specimens than I possess 

 may perhaps determine. 



A shell from the older Tertiaries at Bracklesham, somewhat resembles this in its 

 graceful and elegant form, and slightly so in the disposition of the sculpture, but it has 

 more numerous rays, and the imbricated lines of growth are finer. 



11. PECTEN DUBIUS Brocchi. Tab. IV, fig. 3, and Tab. VI. fig. 3. 



List. Hist. Conch., lib. iii, p. 1, fig. 29, 1687. 



OSTREA DUBIA. Broc. Conch. Foss. Subap., p. 575, t. 16, fig. 16, 1814. 

 PECTEN SCABRELLUS. Desk. 2d ecL Lam., t. vii, p. 161. 



Bast. Bord. Foss., p. 73, 1825. 



Gold/. Pet. Germ., t. ii, p. 62, t. 95, fig. 5. 



Dujard. Mem. Geol. de France, p. 270. 



Bronn. Leth. Geog., ii, p. 917, t. 39, fig. 17, a c, 1838. 



Grateloupe. Cat. Zool. des An. Vert, et Invert. Bord., p. 58, 1838. 



Phil. En. Moll. Sic., vol. ii, p. 60, 1844. 



MURICATUS? Risso. Princ. Prod, de 1'Eur., t. iv, p. 304, 1826. 



VENTILABRUM 1 Goldf. Pet. Germ., t. ii, p. 67, t. 97, fig. 2. 



SCWERBYI. Nyst. Coq. Foss. de Belg., p. 293, pi. 22, fig. 3, b, and pi. 22 bis, 



fig. 3, a', 1844. 



TUMESCENS. S. Wood. Catalogue, 1840. 



