SAXICAVA. 161 



the foot in 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 dwelling in the littoral and laminarian sand- 

 stone rocks, and the other, in the crevices of old bivalves 

 amidst masses of Serpulae. 



S. ARCTICA, Linnaeus. 



-S. arctica, Brit. Moll. i. p. 141, pi. 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 1620 dirty- white short cirrhi; a light 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 

 branchiae 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 in 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 Serpulas 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 



