36o Mr. Osler on burrowing 
the Teredo can bore, as the young Pholades have been de- 
scribed to do, by the action of these muscles alone. 
The posterior adductor is attached to a large process in 
each valve, which, in the common Teredo is a perpendicular 
oval plate, resting against the dorsal margin of the valve, 
and formed by a prolongation and expansion of the tubercle 
at the hinge. About a third part of it is concealed within the 
shell, and the upper margin is rolled outward at its anterior 
end. In the Teredo palmulatus this process is nearly trian- 
gular with the apex to the hinge ; and is formed by an exten- 
sion of the shell itself, of which it is a mere beaklike projection. 
Both these processes are represented in the drawings. 
If all the boring shell fish penetrated mechanically, it 
would be reasonable to expect that their powers should evi- 
dently be in proportion to the hardness of the bodies which 
they inhabit : this is found to be the case in the different spe- 
cies of Pholas ; but the Lithophagi, which would have the 
greatest mechanical resistance to overcome, appear to be de- 
stitute even of the smallest mechanical force. They have 
nothing which in the slightest degree resembles the boring ap- 
paratus of the Pholas. On the contrary, their shell, as in the 
bimusculous Conchiferse, is expanded by a powerful elastic 
ligament, and closed by two large round internal adductors. 
The valves in most of the species shut close, and the foot is 
not an instrument adapted for firm adhesion. 
Four species of Lithophagi are found in the neighbourhood 
of Swansea ; but as the Saxicava rugosa (Mytilus rugosus of 
Linnaeus,) is so abundant, that it may be procured in any 
quantity, I shall take it as the type of the family. 
The general structure of Saxicava rugosa is very similar 
