MAZATLAN UNIVALVES 285 



vol. vii. p. 645, no. 10. Wee. in Zeit. f. Hal. 1851, p. 36, 

 no. 124. (Crypta G.) H. $ A. Ad. Gen. i. p. 369. (Le 

 Jenac,) Adans. Coq. de Sen. p. 41, pi. 2, f. 10. 

 For other references, v. C. B. Ad. loc. cit. 



The late lamented and most accurate Prof. Adams, who in 

 all other instances has created fresh species sooner than allow 

 that the same shell could be common to the Atlantic and 

 Pacific waters, has here, and here only, departed from his 

 theory, and has quoted the above shell from the following six 

 zoological provinces, to which we must now add a seventh : 

 1. Mediterranean 2. East coast of North America, North of 

 Cape Cod. 3. Do. South of the Cape. 4. The Carribean 

 waters. 5. West tropical America. 6. S. W: temperate 

 region. Also fossil in Italy, Morea, Bordeaux, Dax and Tour- 

 aine, Desk. "Fossil nel PiacSntine e nel Sanese," Broc. And 

 yet it is not impossible that in this his only instance, he may be 

 in error ; and that the shells he has quoted from Panama (with 

 those of Brod. from Panama, perhaps from Chiloe,) are varie- 

 ties of his own C. nivea ; as the shells of D'Orbigny, quoted by 

 him under this species, are said by Dr. Gray (B. M. Cat. 

 D'Orb. Moll. p. 49, no. 418,) perhaps not correctly, to be 

 varieties of C. dilatata. The form either of margin or of deck 

 is not to be relied on in shells of this type. 



On examining however the young shells classed as C. nivea, 

 it was found that some of them differed from the rest in the 

 following particulars. 1. The spiral vertex is much larger, 

 being, in a shell '095 long, '025 across. 2. It is smooth, not 

 concentrically wrinkled. 3. It is sunken in the produced mar- 

 gin of the shell, not prominent as in C. nivea. 4. It expands 

 so evenly, that it is difficult to fix upon the point at which the 

 spiral part ends. 5. The large spire shews conspicuously 

 through the columellar lip, (at an angle from the deck, as in C. 

 nivea.) 6. The epidermis appears much thinner. In all other 

 respects, its mode of forming the deck, the shape of its margin, 

 the white colour often streaked with brown, and the tendency 

 to grow in layers, it agrees exactly with the larger species. 

 Whether these characters are peculiar to the shells of the 

 ordinary form Cr. unguiformis, which dwells in dead shells 

 and is therefore liable to transportation through all seas, I 

 cannot tell, as the vertices are rarely perfect, being lost at the 

 advance of the shell. It is however worthy of examination 

 by those who have access to young shells on the Atlantic coasts. 

 That two shells, so very similar and similarly variable in the 



