HYDROBIA. 63 



shells which he had received from Draparnaud under the 

 name of " Cydostoma anatinum." These appear to be 

 a small variety of the present species, and are probably 

 the B. humilis of M. Boube*e. 



Genus III. HYDRO'BIA* Hartmann. 

 H. III. f. 10, 11, 12. 



Eyes placed on tubercles: operculum horny and thin, 

 paucispiral. 



The little mollusks which are comprised in this genus, 

 although very closely related to the true and marine 

 Bissoce, appear to have as much right to be generically 

 separated from the latter as Neritina has to form a 

 distinct genus from Nerita. Nearly all the Hydrobm 

 are inhabitants of fresh and pure water; but one of 

 them (H. ventrosa) frequents estuaries, as well as pools 

 and ditches close to the sea-shore which are liable to be 

 occasionally overflowed by the tide, and the water of 

 which is more or less brackish. Those species which 

 inhabit fresh water have been arranged by Moquin- 

 Tandon in a subgenus ofBythinia, named by him Bythi- 

 nella. With Bythinia, as well as with Bissoa, this genus 

 has undoubted relations. It differs, however, from the 

 former in the eyes not being sessile, but placed on short 

 tubercles, and from the latter in wanting the caudal 

 filament which is appended to the foot. The shell of 

 Hydrobia has besides an umbilical cleft which does not 

 exist in Bissoa; while its operculum is decidedly not 

 Bythinian, but Bissoan, or rather Littorinan. It may 

 therefore be considered as in many respects forming an 

 intermediate link between those two genera, but having 

 a greater affinity to Bissoa. The genus Hydrobia was 



* Living in water. 



