30 AMERICAN JOURNAL 



REMARKS ON THE ANATOMY OF THE GENUS SIPHO- 

 NARIA, WITH A DESCRIPTION OP A NEW SPECIES. 



BY WM. H. BALL, SMITHSONIAN INSTITUTION. 



The genus Siphonaria, described by Sowerby in 1824, is one 

 of the most natural and homogeneous instituted at that early day. 

 It comprises some ninety species, principally from the tropics, 

 but has representatives in most parts of the temperate zone. 

 Probably the most northern species yet described is one men- 

 tioned in this paper — *S'. thersites, Cpr., from Sitka and the 

 coast of Alaska in lat. 57° N. 



If a conchologist were to take a specimen of this species in 

 one hand and in the other a specimen of S. gigas, Sby., from 

 Panama, he would hardly be inclined, however, to place both of 

 them in the same group. One is smooth, horny and minute, 

 with the apex subterminal, and the siphon rib very large ; and 

 the other is very large, solid and heavy, with the apex central, 

 and no outward indication of the siphon. Between these forma, 

 however, we find gradations ; yet the genus, from the shells 

 alone, may be separated into two natural groups, perhaps of 

 subgeneric value, of which one contains the greater proportion 

 of the species. The dentition of a species of Siphonaria (spe- 

 cific name not given) is figured by Woodward in his manual, and 

 has been copied by all subsequent authors, or described (as by 

 H. and A. Adams) as representing the dentition of the genus. 

 The species is from the Cape of Good Hope, and from that 

 locality all the described species belong to the S. sipho group. 

 I have not been able to obtain the soft parts of more than one 

 of the species of that group, to confirm Woodward's figure. 



The examination of several species of the group typified by 

 S. thersites, shows a decided difference in the dentition. 



The following is suggested as an arrangement of the family : 



