MONOGRAPH OF THE GENUS STROPHIA. 
171 
Margin, single, not produced forward quite as far as the diameter 
of the shell, it is thin, a little thicker than the shell behind it, is 
smoothly rounded, and produced backward into a blunt edge, While 
the frontal bar is quite well developed, it does not project much 
beyond the striations. 
Color of shell, externally, pale ashy blue, becoming horn color on 
the apex, and waxy white on the margin, which extends backward 
past the teeth when the shell becomes quite abruptly brown, with 
almost a sharp line of demarkation. 
DIMENSIONS. 
Size of type, 1.07 by .45. Largest specimen, 1.09 by .45; 
smallest, .87 by .37. Longest specimen, 1.09 ; shortest, .87. Greatest 
diameter, .45 ; smallest, .37. 
OBSERVATIONS. 
Individual variation is not great. As might be expected, there is 
some marks of reversion toward the type form, which, as I shall clearly 
show, must have been S. bimarginata, and this reversion takes the 
form which w T e should most expect, that of the double tooth, but this 
is never as marked as in typical S. bimarginata, and out of the 
one hundred and twenty-five examined I have found twenty only, and 
in more than one half of these the central tooth had the slightest indi- 
cation only of the doubling. 
This double tooth is never accompanied by the double margin, 
and the only indication of the feature so prominent in S. bimarginata 
is a slight furrowing of a thickened and beveled margin in some 
specimens. 
There is a decided inclination to produce a form w T ith fewer stria- 
tions, some, in fact, being nearly smooth on the upper whirl. 
There is one decided form, and that is one in which although the 
form has not changed, the color has become reddish ash throughout, 
striations and all. This color appears in intermediate specimens on 
the apex, thence gradually spreads, through other specimens, over the 
entire shell. Some adults are slightly flecked. This is the form 
through Avhich the sub-species next described, S, p. evolva, was 
derived. 
I have named this interesting species of Strophia for Mr. A. II. 
Pilsbry, the accomplished curator of conchology at the Academy 
of Natural Sciences, Philadelphia. 
