5 
the limestones exhibited. The range in height was also 
against their being the work of marine molluscs. He 
thought the perforations were more probably due to atmos- 
pheric agency.” 
According to Dr. Baird, in his Dictionary of Natural 
History/’ the Saxicava is a genus of conchiferous Mollusca, 
nearly related to the Myaclce, or, according to some arrange- 
ments, forming the type of a small family by itself 
Saxicavidce. There is only one species of the genus known, 
but its protean form has camsed conchologists to make, at 
least, two genera of it. When young the shell is symme- 
trical, with two small teeth in each valve. In this state it 
is the Hiatella of authors. When it is full grown it is 
rugose and toothless, forming the true Saxicava . S. rugosa 
is found in almost all parts of the world, and it is found in 
various situations living in the crevices of rocks or corals 
or amongst the roots of seaweed, in which places it is 
attached by a byssus, or boring into limestone and other 
rocks and other shells It assumes so many forms that no 
fewer than 15 species have been manufactured out of its 
varieties and conditions. It ranges from low water to 140 
fathoms, and appears to attain its largest growth in the 
Arctic seas. Unlike the rough rasping shell of the Pholas, 
this little mollusc has its shell smooth and covered 
with an epidermis, and its toot is so small and finger like 
that it possesses none of the power enjoyed by the foot of 
Teredo. It has, accordingly, been supposed by some con- 
chologists that it dissolves the rock by chemical means, 
softening it by some peculiar acid it has the power of 
secreting, so as to be able to wash the debris away by means 
of a current of water passing through the branchiae. By 
others it is asserted that the anterior margins of the mouth, 
which are united and much thickened, are covered with 
siliceous particles, and form a rasping surface sufficiently 
powerful to wear the rock away. 
