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 imposition of a newname. ‘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 im 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. /zearis, only in that shell, neither the valves nor the 
rays are so much depressed. Our shell is ornamented with fine concentric striz, 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 /znearis. 
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. Pecren pusius Brocch. Tab. IV, fig. 3, and Tab. VI. fig. 3. 
List. Wist. Conch., lib. iii, p. 1, fig. 29, 1687. 
Osrrea puBIA. Broce. Conch. Foss. Subap., p. 575, t. 16, fig. 16, 1814. 
PECTEN SCABRELLUS. Desh. 2d ed. Lam., t. vii, p. 161. 
= — Bast. Bord. Foss., p. 73, 1825. 
— — Goldf. Pet. Germ., t. ii, p. 62, t. 95, fig. 5. 
—_— — Duward. Mem. Geol. de France, p. 270. 
—_ — Bronn. Leth. Geog., ii, p. 917, t. 39, fig. 17, a—e, 1838. 
_ _ Grateloupe. Cat. Zool. des An. Vert. et Invert. Bord., p. 58, 1838. 
— — Phil. En. Moll. Sic., vol. ii, p. 60, 1844. 
—  muricatus? Risso. Prine. Prod. de l’Eur., t. iv, p. 304, 1826. 
—  VENTILABRUM? Goldf. Pet. Germ., t. ii, p. 67, t. 97, fig. 2. 
—  Sowersyr. Myst. Coq. Foss. de Belg., p. 293, pl. 22, fig. 3, 6, and pl. 22 bis, 
fig. 3, a’, 1844. 
—  TumEscENS. §. Wood. Catalogue, 1840. 
