and. Shells of Massachusetts Bay. 15 
Description. Body elongated, rounded superiorly, 
very contractile, tapering gradually from the head to the 
tail, which terminates in a fine point, semi-transparent ; 
color generally a rufous brown, the sides dotted with 
numerous wart-like excrescences or papille; im some 
individuals the color is dark brown, with large irregular 
patches of white on the back between the branchize ; 
head short, depressed, orbicular, mouth in the form of a 
horse-shoe, full of strong transverse folds, surrounded by 
numerous papille, and furnished with three cervical 
branchiz on each side, just above the lips; jaws angular 
and corneous. Tentacule two in number, serrated or 
rather transversely sulcate anteriorly, posteriorly com- 
pressed and smooth, contracting into a round sheath, 
which, arising from the back of the head, terminates in 
five unequal, ragged points, the middle or posterior one 
the longest. ‘This sheath is also contractile in itself; a 
small, branchial appendage projects from its posterior 
side, near the base. The dorsal branchiz are distributed 
in five distinct pairs, diminishing gradually in size from 
the first pair, and consist of numerous delicate ramifica- 
tions, springing as it were from a single trunk. "The 
animal has the power to withdraw these, when irritated, 
into its body, so that they appear merely like small 
tubercles or nodules. The foot is broad and white, and 
so diaphanous that the viscera and contents of the intes- 
tinal canal, the latter of a pale red color, are plainly 
discernible. Genital and anal orifices distinct and sepa- 
rate, on the right side; the former, between the tenta- 
cule and the first branchial tuft, is usually closed by a 
sub-conical, apron-like process, projecting backwards 
from its anterior portion, and nearly — the whole 
orifice ; the male organ is filiform, white and | 
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