212 Mr. SowegnBY's Observations 
-Some of the fragments of stone in the possession of Mr. — 
Sowerby contain another species of Mytilus, which evidently in- 
truded itself into the cavities made by the former, for it does 
not fit those cavities, any more than the Ostrez hereafter to be de- 
scribed; this species is the Mytilus bidens (of the Syst. Nat.) 
figured in Chemnitz's Conchilien Cabinet, t. 83. f. 743. In some 
of the cavities are valves of Ostrea Ephippium, of a very diminutive 
size however, and to be considered as a variety of that species, 
for they do not assume the very curved form which gave rise to 
the trivial name. There are also, among these intruders into the  : 
habitations of Mytilus lithophagus, the Mya dübia of Pennant, 
and the Arca reticulatu of Gmelin, besides three other species of 
Testacea, which we at present forbear to make any reportupon, not 
having quite satisfied ourselves whether they be not hitherto un- 
described. Suffice it to say, that one of them approaches to the 
. Ostrea Perna of Linnzus; and there is a species of Venus (of a 
cardoid form), which is rather too much worn and damaged to 
admit of being accurately and perfectly described, but which, 
from appearing to form a cavity for itself in a manner similar to 
the other lithophagous Testacea, deserves to be particularly exa- 
mined. 
Bt -Additional Observations relative to the Ar llophagus 
REE ate ae Sowerby. ` eaters 
^g BEG Nadel to e biii thè Tiap Sodat a desgtiptión 
of the difference that exists between the Mytilus lithophagus and 
the shell of that kind which I. itis had the honour to present 
to the Society. — en 
Gmelin's description of Mytilus lithophagus i is y Biete ** testa 
| "bung tide 
