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 Anc. 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, 



p. 144. 



(23) 



