AND ALLIED FORMS. 23 



chidian tooth, which is quite different in form from that of S. 

 dejjressus, and noticeable for the great length of the central den- 

 ticle of the seven-denticulate cusp, » the basal denticles are three 

 in number, of which the innermost is by far the largest, and projects 

 beyond the basal margin of the tooth ; this basal margin is trilo- 

 bate, as in S. depressus. The intermediate tooth has its peduncle 

 twice as long as the body, and very slender ; its body is perforated, 

 and has a strong projection or shelf at the lower margin of the 

 perforation, of nearly similar shape and size with the perforation 

 itself, and appearing, in some points of view, very much like an 

 open door or lid ; the cusp is armed with about seven strongly 

 prominent, sharply pointed denticles. The inner lateral tooth 

 has twelve denticles at the summit. The outer lateral has about 

 the same number, much smaller, as usual, than those of the inner. 



It will be noticed that this dentition differs in some points from 

 that of the type, S. depressus ; and is remarkable in the length 

 of the rhachidian cusp, as well as in the peculiar process guarding 

 the perforation of the intermediate tooth. These differences 

 would not, however, seem to be generic, as far as can be judged 

 by our present knowledge of the value of such characters in the 

 Hydrobiiuse. 



The shell of S. isogonus is similar to that of S. depressus. 

 The rostrum and tentacles are also similar to those of the type ; 

 they may be seen in Fig. 13. The tentacles, contracted by spirits, 

 are of course much shorter than in life. 



Mr. Currier informs me that S. isogonus has the habits of 

 Vivipamis, except that it burrows more deeply into mud. He 

 finds it only on clay bottoms. 



The Melania Integra of Say, and M. altilis of Lea, and the 

 Faludina subglobosa of Say, are placed in Somatogyrus by 

 Tryou.2 The first two species, however, belong to a different 

 genus, as will be noticed below. The Pahidina subglobosa I 

 have not seen. It is perhaps the same as S. isogonus.- 



' The figure was drawn from a specimen in which the cusp was broken 

 down by pressure ; the lateral margins are in reality continuous. 



2 Proc. Acad. Nat. Sci. Phila., 1864, 104. Mr. T. spells the generic name 

 Somatogyra. 



