144 Transactions. — Zoology. 



the inferior margin more or less trilobate. First lateral broad and ex- 

 cavated in the middle, contracted into a long peduncle, the denticles nearly 

 equal. Second lateral pointed at the inner extremity; the shank broad, and 

 thickened on its outer margin. Third lateral with the inner extremity 

 broad and rounded, constricted at its junction with the very broad shank, 

 which is thickened on its outer margin. Number of transverse rows of 

 teeth, 55 to &Q. 



Formula of the denticles, ^J^^ll^^^ ^; 9 or 11 ; 20 to 23 ; 30 to 40. 



The formula of the denticles differs widely from that of Bythinella, and 

 approaches more nearly those of Stomatogyrus and Amnicola ; but Potamo- 

 pyrgus is readily distinguished from both these genera by the shape of the 

 third lateral tooth. 



P. cumingiana and P. antipodum are both ovo-viviparous, and probably 

 the other species are the same. The species inhabit both fresh and brackish 

 water. They are very variable in form, and the only reliable character is 



the dentition. 



Key to the Species. 

 Shell with spines, whorls more or less carinated. 



Spines long ; first lateral tooth with 9 denticles . . . . P. cumingiana. 



Spines short ; first lateral tooth with 11 denticles . . ..P. corolla. 



Shell without spines, whorls rounded. 



Whorls 5 or 6 ; basal denticles 3-3 . . . . . . ..P. antipodum. 



Wliorls 4 ; basal denticles 4-4 . . . . . . ..P. pupoides. 



P. CUMINGIANA. 



Plate I., figs. A and E. 

 Paludestrina cumingiana, Fischer. Jour, de Conch, viii., 1860, p. 208. 

 Paludestrina salleana, Fischer, I.e. 1860, p. 209 (?). 

 Melania corolla, Keeve. Conch. Icon. fig. 366 ; not of Gould. 



Shell ovate, thin, olive-brown ; whorls 5^ to 6|, angulated ; a row of 

 distant curved spines on the last two or three whorls, 10 to 17 spines on the 

 body whorl. 



Formula of denticles, ^ ; 9 ; 23 to 25 ; 26 to 30. 



Axis, -2 to -23 inch ; breadth, -12 to -1. 



Habitat: The northern part of the North Island and the valley of 

 the Waikato. I have not seen it from Wellington, nor from any part 

 of the South Island. Fresh-water only. 



The shell is very variable in shape, especially in the angle of the whorls, 

 which is sometimes sharp, sometimes rounded, and sometimes absent 

 altogether on the body whorl. The peritreme is usually continuous, but 

 occasionally it is discontinuous in apparently adult shells. The lower side 

 of the median tooLh is trilobate, but the middle lobe is not conspicuously 

 developed, and is sometimes sligiit. 



