HYDROBIA. 63 



shells which he had received from Draparnaud under the 

 name of " Cyclostoma anafinum.'^ These appear to be 

 a small variety of the present species, and are probably 

 the B. humilis of M. Boubee. 



Genus III. HYDRO'BIA *, Hartmann. 

 PL III. f. 10, 11, 12. 



Ei/es placed on tubercles : operculum horny and thin, pauci- 

 spiral. 



The little mollusks which are comprised in this genus, 



although very closely related to the true and marine 



RissocBy 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 Hydrobice 



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 of Bythinia, named by him Bythi- 



nella. With Bythinia, as well as with Rissoa, 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 Rissoa ; while its operculum is decidedly not 



Bythinian, but Rissoan, 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 Rissoa. The ' genus Hydrobia was 



* Living in water. 



