MUSEUM OF COMPARATIVE ZOOLOGY. 401 



The surface of the body is less rugose than in P. Quoyana, though it is very 

 finely granular all over, with (in the contracted specimen) a velvety appear- 

 ance. The head bears about the same proportion to the rest of the body, but 

 the muzzle in front of a line joining the tentacles would be shorter, and the 

 tentacles shorter, stouter, and more pointed. The eyes are very small, black, 

 and similar to those of P. Quoyana. 



The epipodia are similar in arrangement, but proportionately smaller than 

 in that species, and while minutely papillose do not seem to have a special 

 single fringe-like row of papillte at the margin. The right epipodium ex- 

 tends forward as far as the front end of the foot, the left only to the anterior 

 edge of the operculigerous disk. The difference is not nearly so great in 

 P. Quoyana, where the left epipodium extends one half the remaining distance 

 farther forward. 



The most marked external difference between the two animals, as observable 

 from the alcoholic specimens, consists in the character of the dorsal area be- 

 tween the two epipodia. The operculigerous lobe is very much larger in pro- 

 portion (about 33.0 mm. in diameter), while behind it in the median line is a 

 large, deep, ill-defined groove, extending to the posterior end of the foot. 

 This is crossed by irregular strong transverse rugae, which are often tuberculate 

 or warty along what would be the margins of this groove. In P. Quoyana 

 this dorsal depressed area is nearly smooth, while the part behind the opercular 

 lobe, though more papillose than the rest, shows no such median groove or 

 transverse ruga?. 



The foot is shaped as in P. Quoyana ; the anterior margin appears narrowly 

 duplicate, but this, owing to contraction, may be an erroneous deduction. In 

 this specimen the sides stand about an inch in height (95.0 mm.) ; the sole of 

 the foot is about an inch wide, and three inches long. 



The lobes of the mantle correspond to the form of the shell, and are smooth 

 except at the margin, where they are densely papillose, the papillae being 

 small but irregular in size and not arranged in rows, or if regular then in 

 more than one row. This margin extends all round the mantle edge, and on 

 both sides of the notch to its posterior commissure, toward which the papilla? 

 become smaller and sparser. At the end of the commissure a few of the 

 papillae appeared to me at my first examination to be separated from those on 

 either side by a gap, but a second scrutiny leads me to believe that' this is 

 accidental. 



Within the mantle cavity, and on the inner surface of the mantle, rather 

 close to the junction of the latter with the dorsum, are the gills. These con- 

 sist of two series of flattened leaflets on either side of a smooth cutaneous 

 ridge containing the branchial vessels, which extends from a point close to 

 the anterior edge of the mantle, parallel with the slit in the shell, backward 

 as far as the slit extends. A section is given in the diagram on page 434. 

 The anterior end of this ridge is for a short distance free from the mantle 

 surface, and terminates in a sharp point, near and up to which the branchial 

 leaflets diminish in size, the outer seneo extending a little farther than the 



vol. xviii. 26 



