BYTHINIA. 59 



thicker and longer, the whorls being much less swollen, 

 the suture not so deep, the apex or point of the spire 

 more blunt, and the mouth being less circular. Its size 

 is not quite equal to that of the other species, the largest 

 British specimen which I have of this being 1 1 inch 

 long and li broad. 



The animal is rather active. M. Millet counted in a 

 female 82 young ones of different sizes. Mr. Clark has 

 remarked that, in all the specimens which he procured 

 from the River Exe, the point of the spire is eroded. 

 This is probably owing to the influx and admixture of 

 sea-water in that part of the river, because there are few, 

 if any, manufactories on the banks of the Exe. In the 

 Paddington Canal and parts of the Thames near London, 

 the erosion is evidently owing to the last-mentioned cause. 

 Draparnaud named this species Cyclostoma achatinum. 



Genus II. BYTHINIA*, {Bithima) Gray. 

 PI. III. f. 7, 8, 9. 



Eyes sessile: operculum testaceous and solid, irregularly 

 concentric, and having its nucleus nearly in the middle. 



This was first indicated by Dr. Gray as a subgenus of 

 Paludina; and the name which he proposed has been 

 adopted by almost all conchologists. The chief differ- 

 ence between this and Paludina consists in the animal 

 of Bythinia being oviparous, instead of ovoviviparous, — 

 its eyes being sessile, instead of placed on stalks or tu- 

 bercles as in the other genera of this family, — and in the 

 operculum being testaceous and concentric, with its 

 nucleus placed almost in the middle. The tentacles of 

 the male are of equal size in the present genus. Although 

 the derivation of the word Bythinia would imply that 



* Inhabiting deep water. 



