MAZATLAN UNIVALVES 285 
vol. vil. p. 645, no. 10.—Mzke. in Zeit. f. Mal. 1851, p. 36, 
no. 124.—(Crypta G.) H. & A. Ad. Gen. i. p. 369.—(Le 
Jenac,) Adans. Coq. de Sen. p. 41, pl. 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 tothe Atlantic and 
Pacific waters, has here, and here only, departed from his 
theory, and has quoted the above shel! 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. 8. W. temperate 
region. Also fossil in Italy, Morea, Bordeaux, Dax and Tour- 
aine, Desh. *‘ Fossil nel Piacentine e nel Sanese,” Broce. 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 ©. 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 lable 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 
