24 MOLLUSCA FROM THE CRAG. 



them, and as such, they arc here considered only in the Hght of varieties of tlie Hvinf^: 

 British shell. 



M. Nyst, seems to consider the variety ^randis as a modified form of P. Jacohmm, 

 from which opinion I must dissent, believing it to be, as above stated, specifically 

 united vA\\\ our own species P. maximus. In P. Jacoba-us, the costae or rays are 

 broader in proportion to the intermediate depression, and are more quadrate : those 

 ujion 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. complanatm 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 Jacobeeus 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 bii)artite 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 conve.\ 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 Pj/rula, Pholadomi/a, Limjula ; forms, now found only in Tropical 

 or Sub-Tropical Seas, while at the same locality are numerous mdividuals of species, 

 whose homologues are living at the present day upon the Scandina^^an Coast. 



The range of this species {maximus), in the Uving 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 s))caks of it also 

 as from the Mediterranean. 



2. Pecte'n GERAunii, Ni/st. Tab. V, fig. 5, a — b. 



PECTt.N Gekardii. Nyst. Rech. Coq. Foss. Prov. d'Anv., p. 19, No. 75, pi. 3, fig. 75, 

 1835. 

 _ _ Potiez. et Mich. Cat. des Moll, de Doiini, t. 11, p. 78, No. 32, 1S44. 



— — Nyst. Coq. Foss. dc Belg., p. 300, pi. 18, fig. 11, 1S4J. 



— suBDiAPHANi's. S. Wood. Catalogue, 1840. 



Spec. Char. Testa orbiculari, subhyalind, inacpdvahi, equilaterali, radiatim striata, 

 et divaricatim insculptd ; auriculis incequalibus ; maryine tenuissime crenulatd. 



