28 RESEARCHES UPON THE HTDROBIIN^ 



between that of Somatogyrus and that of Byihinella. The rha- 

 chidian tooth is of the usual shape, and has on each side but two 

 distinct basal denticles, exterior to which there is a lobe. These 

 denticles are, however, rather large, and acute, extending beyond 

 the basal margin of the tooth. The cusp of the rhachidian tooth 

 is armed with nine acute denticles. The intermediate tooth has 

 its peduncle rather longer than the body, and the body has a de- 

 pression upon its anterior surface but not a foramen ; its cusp 

 has eight denticles. The denticles of the summit in the inner 

 lateral tooth are fourteen in number, and in the outer one ten. 



The ova-capsules (Fig. 19, c and d) are small, nearly hemi- 

 t;plierical, attached by the broad base, and are deposited singly, 

 or in groups or linear series. Each contains but a single q^^. 



The only known species besides the type G. altilis, which may 

 probably belong to the genus now under consideration is the 

 3Ielania inteyra of Say, found in the tributaries of the Missis- 

 sippi, the shell of which is very similar to that of G. altiUs. A figure 

 of lingual teeth, said to be of this species, given l)y Troschel,* is 

 here copied Tig. 21). The only essential diflerence betweeu 



Fig. 21. 



this dentition and that of G. altilis figured above, is that the 

 rhachidian tooth of the Integra is represented as having but one 

 basal tooth on each side, but this diflerence is an important one, 

 and it is desirable that the teeth of both species should be re- 

 examined on this point. The cavity in the body of the inter- 

 mediate tooth, described above as occurring in G. altilis, is slight 

 and may have been overlooked by Troschel in the other species. 



It is not improbable that two or more species have been con- 

 founded under the name of Amnicola altilis; if so, it is only 

 necessary to state that my observations were made upon the form 

 found so abundantly in the Delaware River near Burlington, 

 which must be considered as the type of the genus. 



' Op. supra cit., I, pi. viii, fig. 4. 



