2°6 DIMYARIA.—GASTROCH ENID 
hinge furnished at one period of life with cardinal 
teeth. The ligament is external and projecting. 
The animal is oblong, with the mantle closed, 
except in front, where exit is afforded for a minute 
finger-like foot, furnished with a byssal groove 
The siphons are united, and their orifices, which 
are large, are margined with a fringe of simple 
filaments. 
SAXICAVA 
“The Saxicave,’ observe the authors of the 
British Mollusca, “ are borers, although the habit 
of boring does not seem necessary to their exist- 
ence, since we find them very commonly free. If 
there be a crevice, however, in rock, shell, coral, 
or sea-weed, into which they can thrust themselves, 
they do so; and if near a limestone rock, perforate 
it, and form crypts, in which they live. Mr. Osler 
states that, when young, they are very active 
