THE NATURAL HISTORY SURVEY. 321 



neath; there is a black patch on the top of the rostrum and 

 about the eyes, and the sides of the foot and body are dark, 

 mottled with light yellow; there is also a yellow band from 

 the back of the eyes to the shell; mantle dark or blackish. 

 Operculigerous lobe conspicuous; head prominent, with large, 

 somewhat elongated rostrum, subconical in form, which pro- 

 jects far in advance of the foot, particularly when the animal 

 is in motion; on the upper surface it is spirally wrinkled when 

 the animal is at rest, but is almost smooth when the animal is 

 in motion; the mouth is placed at the tip of the rostrum, which 

 is disk-like, and is represented by a long, longitudinal slit, 

 which makes the snout-end look like a double disk; the radula 

 may be plainly seen in the mouth when the animal is feeding; 

 tentacles of medium length, somewhat tapering; eyes black, 

 placed on prominences at the outer bases of the tentacles. 

 Mantle simple, folded on the right side to form the respiratory 

 cavity. Generative duct on right side, opening at the junction 

 of back with mantle, the duct is composed of two laminae 

 placed close together; anus placed above the generative duct. 

 The gills (branchiae) are two in number, placed, as usual, upon 

 the under side of the mantle, and are composed of a primary 

 (comb-like) and secondary (feather-like) gill; the former being 

 very long and gradually narrows toward the posterior end, and 

 the latter very simple and running parallel with the primary 

 gill, and composed of a midrib, with bare indications of the 

 "feathers." In one specimen the foot measured 6.00 mill, in 

 length and 3.50 mill, in width.* 



Jaw: Elongately ovate, the surface covered with numer- 

 ous small, rounded or polygonal scales. 



Radula formula: T V + i+i+^+t+t+Ar (3-> 3): cen- 

 tral tooth with a low, wide base of attachment, the reflected 

 portion seven-dentate, the central cusp very large, side cusps 

 very much smaller; intermediate tooth almost square, five- 

 dentate, the inner cusp small, the next cusp very large and the 

 three outer side cusps small; lateral teeth more or less sole- 

 shaped, longer than wide, the first eight- and the last ten-den- 

 tate; the outermost lateral flares a little at the upper part. In 

 one membrane the writer counted 43 rows of teeth. The teeth 

 are in all respects like those of P. elevatum. 



Genitalia: Not examined thoroughly. The sexes are dis- 



*The writer has been very materially assisted in making the anatomical examinations 

 of this group by an article by Dr. William Stimpson, in Amer. Journ. Sci. and Arts, 2nd 

 series. VoL XXXVIII, p. 41, 1864. 



