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 Os¢. 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 strie. 
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 strize 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, J. Sowerby. Tab. VI, fig. 5. 
PecTtEN GRaciLis. J. Sow. Min. Conch., t. 393, fig. 2, 1825. 
_ — S. Wood. Catalogue, 1840. 
— — J. Morris. Cat. Brit. Foss., p. 114, 1843. 
Spec. Char. Testdé gracili, suborbiculari, compressa vel planiusculd, tenui, costatd ; 
costis acutis, angustis, inequalibus, tripartitis, concentricé et tenuissime imbricato-striatis ; 
auriculis parvis inequalibus. 
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, 14 inches. 
Locality. Red Crag, Sutton, Holywells, and Bawdsey. 
