286 MOLLUSCA FROM THE CRAG. 



SAXICAVA PHOLADIS. Turt. Brit. Biv., p. 21, t. 2, figs. 11, 1822. 



Hancock. Mag. Nat. Hist., vol. xviii, p. 337. 

 DISTORTA. (Say.) Gould, Inv. Massach., p. 61, fig. 40. 



STJLCATA. Smith. Phil. Trans., 1835, pi. 2, fig. 25. 

 PHOLEOBIA RUGOSA. Leach. Ross's Voy. Baff. Bay, p. 174, 1819. 

 BIAPHOLUS RUGOSUS. Leach MS. An. and Mag. Nat. Hist., vol. xx, p. 272. 

 BYSSOMYA PHOLADIS. Bowdich. Bivalves, fig. 43. 



RHOMBOIDES RUGOSUS. Blainv. Man. Malac., p. 573, 1825. 

 HIATELLA OBLONGA. Turt. Brit. Biv., p. 25, pi. 2, fig. 13, 1822. 

 RUGOSA. Flem. Brit. An., p. 461, 1828. 



Spec. Char. Testa variabile, oblongd, vel subrhomboidali transversim striatd, rugosa ; 

 utraque ecctremitate obtusd, aliquando valvarum angulis binis instruclo: latere antico 

 brevissimo. 



Shell variable, oblong or subrhomboidal, transversely striated, and rugose ; obtuse 

 or rounded at each extremity ; sometimes furnished with two diverging rows or slightly 

 imbricated ridges ; anterior side much the shorter. 



Length, 1 inch. Height, f inch. 



Locality. Cor. Crag, Sutton. 



Red Crag, Sutton, Walton Naze. 

 Clyde Beds, Bridlington. 



Recent, Britain, Mediterranean, Scandinavia, N. E. Coast of America, 

 Nova Zembla, Sea of Ochotsk, and Sitka. 



Small specimens of this shell are abundant in the Cor. Crag. In the Red Crag 

 they are sometimes met with, in loose sand, with the valves united, much distorted, and 

 with a large, ventral opening. The gigantic specimens obtained in the Clyde Beds, 

 and in the recent Deposits of Canada, belong, I believe, to nothing more than a variety 

 of this species, where, apparently under favorable circumstances, it had attained to so 

 great a magnitude ; but one of my specimens from the Red Crag is not much less, 

 giving every reason to suppose the simple difference in size, if not merely a difference 

 in age, may be the result of different conditions ; the Red Crag specimens bearing in 

 general a sort of intermediate character, as if a reduction in temperature from the 

 older to the more modern Periods had been favorable to the fuller development of 

 this species. 



Large numbers of individuals are found loose in the Crag, and when in a living 

 state, probably passed their lives in adhering by a byssus to the roots of Fuel. Mr. 

 Sowerby, in ' Min. Conch./ speaks of a specimen having been found imbedded in 

 Septaria, beneath the Red Crag at Holy wells. I have never met with the British 

 fossils otherwise than in localities where, I believe, they were not excavators. This 

 species appears to be generally distributed through the Drift Beds in this country, and 

 it is also found in similar Deposits in Canada, Sweden, and Russia. Mr. Smith, of 

 Jordan Hill, has recently forwarded to me the drawing by the late Professor E. Forbes, 



