ANCYLUS. 229 
16. ANCYLUS FRAGILIS, Tryon. 
Plate 2, fig. 17, 18. 
Shell very small and fragile, sides nearly 
parallel or slightly incurved in the middle, 
but diverging anteriorly; ends rounded. Apex 
elevated, acute, curved backwards, with about 
two-thirds of the shell anterior to it. 
Length 4, width 1°5, alt. 1 mill. 
California. 
This species is smaller, thinner, and wants 
the convex lateral margins of Ane. rivularis. 
It agrees with that shell, however, in the 
greater width of its anterior end, while in the 
shape of the lateral margins it resembles A. 
parallelus, Hald. It is much the smallest of 
our species, most specimens being only two- 
thirds the dimensions given above. 
Ancylus caurinus, Cooper, mentioned by 
name only, neither described nor figured, is a 
synonym of this species—vide the figure from 
a type specimen in Mr. Binney’s work, f. 243, 
. 144. 
(23) 
