MUSEUM OF COMPARATIVE ZOOLOGY. 229 



pallial line, the region of which is slightly raised, the shell falling away from 

 it toward the centre and the periphery ; outside of the pallial line smooth or 

 marked by irregularities of growth ; inside with fine radiating wrinkles (due to 

 the papillae on the mantle-edge which are strongest at their distal ends, and 

 which there form a narrow band of elevated waves and grooves which borders 

 the body cavity of the shell and is strongest near the hinge); muscular impres- 

 sions distinct, the double impression of the adductor more duplex in the right 

 valve than in the other, the two parts of the muscle being slightly twisted one 

 upon the other ; the anterior adductor slightly nearer the beak than the other ; 

 margin partaking of the irregularities of the object to which the shell is at- 

 tached, usually sharp, simple and very thin, as in a young oyster. Lon. of 

 shell 10.5, lat. 12.0 mm. 



Soft Parts. Mantle open throughout; its margin thickened, smooth, white, 

 rather distantly studded with a single line of small (in their contracted state 

 strawberry-shaped) papillae, more elongated toward the middle; there are no 

 ocelli, but the central portions of the papillae often show a pronounced dusky 

 tinge ; within the line of the papillae a thin smooth " curtain " or band of tissue 

 at right angles to the plane of the shell extends toward the opposite valve, 

 with a width, in its contracted state, about equal to the thickened marginal 

 band; inner portions of the mantle thin translucent and studded with aggre- 

 gations of more opaque whitish cloudy dots or nebulous markings ; the -mar- 

 gin of the mantle has its extreme edge brownish near the anal end, and is 

 somewhat firmly attached to the shell. The adductors are composed of strong, 

 stiff', unstriated, easily separable fibrillae, of a greenish cream-color and very 

 polished surface, and are provided with a thin but stout investing fascia; as 

 before stated, the posterior adductor is composed of two subequal rounded 

 portions slightly twisted on each other as if to admit of a slight lateral motion 

 of the upper valve upon the lower one ; they are quite round except where in 

 contact ; the anterior adductor, with an area equal to one of the halves of the 

 posterior muscle, is more elongated and pointed toward its outer end ; the ali- 

 mentary canal is small, apparently simple, with one certainly and possibly 

 two coils buried in the liver; the oral aperture, just below and behind the 

 anterior adductor, presents a curious resemblance in its form to the epistome 

 of a polyzoan ; the form is not absolutely the same (owing to the alcohol ?) in 

 diff'erent individuals, and in the largest one examined was much more pro- 

 nounced than in smaller examples ; there is first a soft ovoid prominence at 

 whose apex projects from a depression a linguiform lip or projection over 

 which is a waved fold with a deep sulcus between ; from the lateral ends of 

 the " lip " and fold a wrinkle extends forward and then backward, the two 

 wrinkles on each side parallel with a shallow depression continuous with the 

 sulcus between them; the oral aperture appears at the bottom of the sulcus 

 and will receive the end of a probe as large as a good-sized pin, but the course 

 of the oesophagus, being lacerated, could not be continuously traced; the lip 

 and fold in the oldest example dissected appeared to have a hard, almost 



