SAXICAVA. 161 
the foot im S. rugosa is attached to the body by a globular 
base, whereas in S. arctica the pedicle is more linear. The 
valves of the shell in S. rugosa are, though often distorted, 
usually symmetrical, rarely locking into each other; and the 
habitat of the two, at least at Exmouth, is essentially different, 
the S. rugosa dwellmg in the littoral and laminarian sand- 
stone rocks, and the other, in the crevices of old bivalves 
amidst masses of Serpule. 
S. arctica, Linnzeus. 
S. arctica, Brit. Moll. i. p. 141, pl. 6. f. 4, 5, 6. 
Solen minutus, Auctorum. 
Animal subcylindrically elongated, body mottled white ; 
mantle thick, but not so fleshy, nor wrinkled into muscular 
folds, as in S. rugosa, of a pale yellowish-white ; closed, except 
an opening for a narrow strap-shaped foot with a byssal 
groove and obtuse termination. The siphons, when extended, 
are longer than the shell, of a pale yellow at their base, and 
orange at the extremities; they are soldered to each other to 
a short distance from their terminations; they then separate, 
by the anal diverging and curving upwards, and frequently 
protruding a globular transparent valve; the orifices of both 
are encircled by 16-20 dirty-white short cirrhi; a ight brown 
epidermis covers the ventral and dorsal margins, and is con- 
tinued on the siphons to their termination, giving their really 
yellow colour the aspect of a brown skin, which, however, 
appears to be an adscititious deposit, and not an extension of 
a true mantellar membrane that is reflexed on the shell. The 
branchiz-are pale brown, narrow, hung horizontally, the upper 
ones are the smallest ; these are accompanied on each side by 
a pair of very small triangular palpi, of pale yellow hue. 
This species, I believe, is not strictly a borer; it is very 
rarely found in company with S. rugosa im the triassic sand- 
stone, in which the latter abounds; though often taken free, 
it is generally nestled and fixed by its byssus in the interstices 
of the various masses of Serpule that clothe the surfaces of 
old bivalves and other marine substances. At Exmouth it is 
always taken in deep water in the coralline zone. The right 
M 
