160 



increasing enclosed excavations of the Gastrochana 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 intestine, 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 canals of the tubes, those organs being first withdrawn, and 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 BeUevue. 



S. RUGOSA, Linnaeus. 

 S. rugosa, Brit. Moll. i. p. 146, pi. 6. f. 7, 8 ; (animal) pi. 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 f 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 -^ihs 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 Gastroch&na 

 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 branchiae, palpi and byssus of this species are so similar 

 in S. arctica, which follows next, that we refer to it; but 



