64 
LAND AND FRESH-WATER MOLLUSKS. 
black-brown colour. The teeth are seven in 
number; the central with a base produced 
into a horn, with five to seven pointed lobes; 
the first lateral tooth with seven lobes; the 
second is slender, claw-like, and serrated; the 
third is rounded at the tip, with minute denticu- 
lations.— Loven. 
In company with the last species are two other 
brackish-water shells, belonging to the allied 
genus Hydrobia, species of which are more par¬ 
ticularly marine. Hydrobia differs from Assiminia 
in having the eyes placed on tubercles, and from 
Bissoa in its smooth shell. 
The form of the shell of one species, Hydrobia 
similis (PI. II., fig. 7), resembles that of Bithinia 
Leachii , but is distinguished by its smaller size 
and grooved suture; the operculum is horny, 
concentric, with the nucleus lateral; whereas in 
Bithinia it is somewhat shelly, and marked by 
concentric ridges having the nucleus central. 
A second species, Hydrobia ventrosa, is 
closely allied to H. ulvce , whose habits are more 
marine, from which it differs in being half the 
size, with a deeper suture; from H. similis in its 
longer spire, and the absence of the channelled 
suture, and by its much smaller umbilicus. This 
latter species is very abundant on many parts of 
our coasts, in estuaries and in brackish water, or 
upon the mud banks of tidal rivers. 
