12 



RESEARCHES UPOX THE HYDROBIIN^ 



been already briefly alluded to, viz. (1) the Hydrobiinse, charac- 

 terized by their purely aquatic haljits, gliding motion, and the 

 smooth sides of the foot, of which we find the genera Bythinella, 

 ]\loq.-Tand., Amnicola, G. & H., Gillia, n. g., Soinatogyj-vs, Gill, 

 and Fhcminicola, n. g., in the fresh waters of the United States; 

 and (2) the Pomatiopsinoe, characterized by their terrestrial, or 

 more properly amphibious, habits, stepping method of progression, 

 and sinuated sides of the foot, with but one genus, Pomatiopsis. 

 I will now i^roceed to describe in detail such species of these 

 two groups as have come under my observation, reserving the 

 systematic diagnoses of the genera for the conclusion of the paper. 



I. ON THE AMEPJCA?^ FRESH-WATER HYDROBHNi:. 



The considerations which have guided me in the selection of 

 the family name, have been detailed on page 5. 



Genus AMjVICOLA, Gocld & Hald. 



The following description of the characters o? Amnicola proper 

 is based upon the examination of a subglobular species (fig. 1) 

 found in the Potomac Pviver, which is probably 

 the Paludina j^orata of Say, and undoubtedly 

 congeneric with Gould's Amnicola porata, which 

 Haldenian considers to be the Paludina limosa 

 of Say, and which is the type of the genus, 

 being the first species ever referred to it. 



The foot (fig. 2, a, b) is simple, without any 

 lateral sinuses, and is, when extended in progres- 

 sion, between two and three times as long as 

 broad. It is dilated and auricled in front, somewhat narrowed 

 before the middle, more or less expanded behind the middle, and 

 rounded or subtruncate at the posterior ex- 

 tremity. The auricles are pointed, and gene- 

 rally project but little be3'-ond the lateral mar- 

 gin ; but under certain circumstances the animal 

 can elongate them to such a degree that they 

 become almost tentaculiform, and at least as 

 long as the foot is wide. This is represented in 

 the cut fig. 2, b. On one occasion I have seen 

 the auricles thus remarkably protruded, and 



