OF CONCHOLOGY. 149 



deep, obscurely carinate ; umbo produced incurved, forming a 

 nearly complete volution. Left valve nearly flat, but slightly 

 vaulted at the umbo, which is incurved some distance from the 

 margin, but not externally elevated ; tooth wide and strong. 

 Anterior tooth of the right valve large, corrugated on the outer 

 side ; posterior tooth small, corrugated on the inner side. Liga- 

 ment stout, almost buried beneath the incurved umbones. An- 

 terior muscular scars largest, shell channelled between the scars. 

 Interior white ; margins smooth, the posterior edge of the left 

 valve with a narrow band of purple. Exterior without plica- 

 tions, smooth, except for the lines of growth, covered with a 

 strong light yellow brown epidermis, which is only roughened by 

 the lines of growth. 



Alt. 3-1 ; lat '21 in. Depth of right valve, 1*3 in. 



Habitat. — Middle America. " Puget Sound," Rowell, in 

 error. 



This very singular shell is covered with parasitic Vermeti, &c, 

 of Central American species. An anomaly appears in the fact 

 that a large part of the shell, outside of the epidermis, is covered 

 with a white, calcareous deposit, evidently a part of the shell 

 itself, and marked only by coarse lines of growth. The edge of 

 the epidermis appears on the margin some distance within the 

 actual edge. This is the only instance known to me of an epi- 

 dermis being secreted by the mantle beneath the external surface 

 of the shell. It may be accounted for as follows : The edge of 

 the epidermis may be secreted at one period, after which the 

 edge of the mantle might secrete a calcareous layer extending 

 beyond the epidermal margin. The mantle, contracted in 

 a period of rest, would again add to the layer of epidermis, after 

 which, expanding, another outreaching calcareous layer might be 

 deposited, which would, of course, fall upon the previous one, 

 and so on. This view is strengthened by the fact that a careful 

 examination of the epidermis with a strong lens, in places where 

 the external calcareous layer has been removed, shows that the 

 continuity of the epidermis is frequently interrupted by a very 

 thin layer of shelly matter. The outer calcareous layer does 

 not cover the whole of the shell. In any case the fact is 

 very curious and interesting. It is nearest Chama iostoma, 

 Conrad, from the Sandwich Islands, which is a dextral shell 

 with radiating plicie and marginal produced laminae. 



I find a note upon the label, by Dr. Carpenter, stating that 

 he had written, but afterwards suppressed, a diagnosis of the 

 shell under the name of 0. inermis, and, therefore, have adopted 

 his MSS. name, while incurring the responsibility of describing 



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