160 GASTROCH ANID A. 
increasing enclosed excavations of the Gastrochene deposited 
in limestone. We can only suppose, that as additional space is 
wanted, it is procured by attrition, and the spoil either dis- 
charged, when swallowed, through the imtestine, or the shelly 
canals of the tubes, those organs being first withdrawn, and 
placed in a position to clear it out by the ejection of a current 
of water. 
SAXICAVA, Fleuriau De Bellevue. 
S. rugosa, Linneeus. 
S. rugosa, Brit. Moll. i. p. 146, pl. 6. f. 7, 8; (animal) pl. F. f. 6. 
Many conchologists are of opinion that S. arctica and this 
species are identical ; we do not concur with them, as, in our 
opinion, even the conchological indices sufficiently establish 
specific distinction: the examination of many of both of the 
same size, say ?ths of an inch, shows that there are organic 
grounds for specific separation. The tubes of S. rugosa do 
not diverge so much, the cirrhi of the orifices are more simple 
and slender, and the sutural line of the siphons is less pro- 
nounced. When the present species is in sea-water, it opens 
the valves of the shell to the extent of at least ;4,ths of an 
inch, but a similar-sized S. arctica only ~;th: this circum- 
stance is occasioned by the greater mass of folds and rugosi- 
ties of the mantle in S. rugosa, which act as springs to enable 
the animal to open the valves as wide as in Gastrochena 
modiolina, of nearly similar structure and habits. The uses of 
these wide openings are to afford the two last-named species a 
greater ventral mantellar surface to hold the siliceous parti- 
cles, or fine sand imbedded therein, to rub down their stony 
habitations; but in S. arctica, which is a very doubtful 
borer, the mantle is less thickened ventrally, and will not 
allow of anything like the extension noticeable in S. rugosa. 
We have carefully examined the surprising difference of the 
muscular thickness of the mantle in the two species, and have 
found it constant. 
The branchiz, palpi and byssus of this species are so similar 
in S. arctica, which follows next, that we refer to it; but 
