24 MOLLUSCA FROM THE CRAG. 
them, and as such, they are here considered only in the light of varieties of the living 
British shell. ' 
M. Nyst, seems to consider the variety grandis as a modified form of P. Jacobeus, 
from which opinion I must dissent, believing it to be, as above stated, specifically 
united with our own species P. maximus. In P. Jacobeus, the coste or rays are 
broader in proportion to the intermediate depression, and are more quadrate: those 
upon the Crag shell are often as much elevated, but always more or less rounded, 
without the abrupt or sharp edges, which distinguish the Mediterranean shell. The 
most prominent character in the var. grandis, is the distinct ray in the centre of the 
depression, while a corresponding kind of sulcus or furrow runs down the centre of the 
large ray, dividing it into two parts ; this character, which in some specimens appears 
so strongly marked, that it alone would be quite sufficient for specific distinction, be- 
comes in var. complanatus scarcely discernible with the rays very much depressed, and in 
those specimens with this division in the rays, each side appears to be again divided 
by a less distinct line, or arranged in pairs. P. medius. Chemn. is said, by its author, to 
be intermediate between /Jacobeus and Maximus, partaking, in some degree, the characters 
of both, but from the figure and description appears to be only a variety of the latter. 
A species from Australia, somewhat resembles our Crag fossil, in having the inter- 
mediate small ray, but it has no bipartite division of the large rays, while they are 
rounded and simple on the flatter or upper valve, and may, therefore, only be looked 
upon as the representative of our species. In the young state, our shell is nearly 
smooth, while the upper or left valve is concave on the upper surface, in which stage of 
its existence it was probably furnished with a byssus, as an opening is then visible beneath 
the auricle of the convex or right valve, but entirely obliterated in the adult shell. 
In the Coralline Crag at Ramsholt, many beautiful specimens have been found with 
the valves united, in what was, probably, a deeper portion of the sea at that period, 
in association with Pyrula, Pholadomya, Lingula; forms, now found only in Tropical 
or Sub-Tropical Seas, while at the same locality are numerous individuals of species, 
whose homologues are living at the present day upon the Scandinavian Coast. 
The range of this species (mawimus), in the living state, is given by the authors of 
the ‘ Hist of Brit. Moll.,’ from the Coast of Norway to Gibraltar. Payraudeau quotes 
it as found, though rarely, on the West Coast of Corsica; and Born speaks of it also 
as from the Mediterranean. 
2. PectEN Gerarpu, Wyst. Tab. V, fig. 5, a—d. 
Prcten Gurarpit. Myst. Rech. Coq. Foss. Prov. d’Anv., p. 19, No. 75, pl. 3, fig. 75, 
1835. 
— = Potiez. et Mich. Cat. des Moll. de Douai, t. 11, p. 78, No. 32, 1844. 
= = Nyst. Coq. Foss. de Belg., p. 300, pl. 18, fig. 11, 1844. 
—  suppiaPpHANus. S. Wood. Catalogue, 1840. 
Spec. Char. Testa orhiculari, subhyalind, inequivalvi, equilaterali, radiatim striata, 
et divaricatim insculptd ; auriculis inequalibus; margine tenuissime crenulatd. 
