16G 
MONOGRAPH OF THE GENUS STROPIIIA. 
pale brown, a little darker in shade well within the shell, and 
becoming white on the margin. 
DIMENSIONS. 
Size of type, 1.07, by .45. Largest specimen, 1.20 by .55 ; small- 
est, .80 by .35. Greatest diameter, .55; smallest, .35. Longest 
specimen, 1.35; shortest, .80. 
OBSERVATIONS. 
Individual variation is considerable and extends in several 
directions. The form is sometimes shorter and wider proportionally, 
much as in S. glans, and the margin is thinner and less inclined to be 
double, but is never beveled as it is, and differs from S. glans, in main- 
taining the small rounded aperture. Some specimens are also more 
cylindrical and smaller in diameter. The double tooth occurs only in 
perfectly adult specimens and is not always present then. This 
would also appear to be a character which seems strongest in indi- 
viduals which are placed in exposed situations, and thus now is rather 
of a secondary specific character, but as the species becomes more 
fixed, the double tooth will become more constant ; it is present, how- 
ever, is about seventy-five per cent, of the specimens obtained on Green 
Key, the locality of the t} r pe, and in about 50 per cent., or a little less, 
of the specimens from Little Galden Key. 
The double margin of this species is a singular character, and one 
which I have not seen in any other species of Strophia. It is present 
in a more or less marked degree, in nearly all adult shells, but some- 
times the doubling is represented by lines of growth on a simple 
grooved and beveled edge ; possibly, even in these cases, there will be 
another margin formed. In the example given in Fig. 52, d, there 
has been two distinct margins formed, both beveled, grooved, and 
marked with lines of growth, a significant fact and one to which 1 will 
refer again. 
In this connection it is worthy of remark that I have a single 
specimen of S. glans in which two distinct margins have been formed, a 
considerable distance apart, but 1 shall figure this specimen and refer 
to it later. 
The doubling of the margin and of the teeth in this species 
appears to have sometimes caused reversion to some long past 
ancestral type, which was the common ancestor of all the sub-genera 
of the genus, for in this species we find that several teeth appear in 
