BIVALVIA. 37 



long before they attain that size, the separation of the rays generally producing a 

 corresponding ornament upon the intermediate spaces, although that often depends 

 upon the width of those spaces. This appears to differ from the London Clay shell 

 and I doubt its being Ost. recondita, Brander. 



In my specimens from Barton, are two varieties, one not having more than 18 or 20 

 rays, while the other has 30 32, these are more rounded than in the Crag shell ; when 

 perfect, in general they are more or less eroded, they are striated both upon and between 

 the rays, but never distinctly keeled, the imbricated lines of growth are finer, and the auri- 

 cles are comparatively larger. Nor could I detect upon them the fine divaricating stride. 



The auricles of our shell may be described as unequal, that of the left valve on the 

 anterior side projecting, so as to form an angle less than 90. In the right or flatter 

 valve, the anterior auricle is longer or projects further than the posterior one, leaving 

 in the full grown shell a considerable opening, even in those which have exceeded two 

 inches and a half, and on the edge of the shell on that side are prominent denticles' 

 four or five of which may be seen in the open space beneath the auricle. 



The height of most specimens measuring from the umbo to the ventral margin, is 

 rather less than the diameter of the opposite direction, although in some specimens 

 these dimensions are reversed. The angle formed by the divergence of the rays from 

 the umbo may be called a right angle, although it sometimes exceeds, while in the 

 more elongated specimens, it falls short of that size. In some old shells, the anterior 

 and posterior sides are extended, so as to give a high-shouldered appearance to the 

 valve ; and in all the younger specimens, the comparative size of the auricles is greater 

 than in the larger shells, indeed, in the pullus state, they are equal to the entire 

 length, and the shell then is nearly smooth. 



Under a lens, fine divaricating striae may be seen diverging or curving over the shell. 



This species is stated by the authors of the ' Hist. Brit. Mollusca,' to possess a 

 great vertical range, from five to one hundred fathoms, while its peculiar province in 

 the British Seas is between fifteen and twenty-five. 



10. PECTEN GRACILIS, /. Sowerby. Tab. VI, fig. 5. 



PECTEN GRACILIS. J. Sow. Min. Conch., t. 393, fig. 2, 1825. 

 S. Wood. Catalogue, 1840. 

 J. Morris. Cat. Brit. Foss., p. 114, 1843. 



Spec. Char. Testa gracili, suborbiculari, compressd vel planimctdd, tenui, costatd ; 

 costis acutis, angustis, inagualibus, tripartitis, concentrice et tenuissime imbricato-striatis ; 

 auriculis parvis intequalibus. 



Shell suborbicular, compressed or flattened, delicate, thin, and fragile, costated, 

 ribs sharp and fine, unequal in size, arranged in threes ; finely striated concentrically, 

 with imbricated and slightly elevated lines of growth, ears small and unequal. 



Diameter , 1^ inches. 



Locality. Red Crag, Sutton, Holywells, and Bawdsey. 



