FAMILY SAXICAYID^ — SAXICAVA. 227 



Saxicava DISTORTA. 



PLATE XXXIII. FIG. 309. a. b. 



(STATE COLLECTION.) 



Saxicava distoTta. Say, Journ. Acad. Nat. Sciences, Vol. 2, p. 318. 

 •S. id, Gould, Invertebrata of Mass. p. 61, fig. 40. 



Description. Shell thick, coarse, transversely ovate-oblong, inequivalve, irregular in shape 

 and often distorted, generally rounded in front and more or less truncated behind, often with 

 a prominent rounded ridge passing from the beaks to the lower angle, and which is some- 

 times roughened with scales. Beaks rather prominent, and on the anterior third. Surface 

 roughened and undulated by the different stages of growth. Basal margin irregular, usually 

 contracted in tlie middle, with a silken appendage issuing from it. In young specimens, a 

 Blight rudimentary tooth in one valve is received into a cavity in the other, but both disappear 

 with age. 



Color. Epidermis light ashen grey : foot bright orange. 



Vertical axis, 0*4 -0-6 ; transverse ditto, 0-7 - I'O. 



This shell is found along the whole coast, adhering to marine bodies, and is so irregular 

 tliat scarcely two specimens can be found alike. It is often found imbedded in Sponges and 

 among Ascidea. The S. rugosa of Turton {Conch. Ins. Brit. p. 20, pi. 2, fig. 10), with 

 which this has sometimes been confounded, is more transversely elongated, the beaks more 

 central, the elongated side more abruptly truncate, and the dorsal margin more sloping ; the 

 surface furrows are subquadrate following the truncation of the elongated side. 



GENUS PETRICOLA. Lamarck. 



Mantle with its borders simple, slightly dilated in front, where there is a small opening for 

 the passage of a feeble tongue-shaped foot. Tubes small, conic, truncate at their sum- 

 mits, separated for two-thirds of their length, and minutely radiated at their orifice. Gills 

 small. Shell transverse, inequilateral, rounded before, narrowed posteriorly : hinge ahnost 

 toothless ; ligament exterior. 



29* 



