A. JEJ. Verrill — North American Cephalopods. 269 



state when the animal died. Caxidal fin t]-ansversely rhoraboidal, 

 or broad spear-shape, about one-third wider than long; its breadth 

 usually less than half the length of the mantle; the posterior borders 

 are nearly straight and form nearly a right angle at the posterior 

 end ; the anterior mai'gins are somewhat convexly rounded, and the 

 front margin extends, at the sides of the body, considerably forward 

 beyond the insertion of the fin. Ratio of fin-length to mantle-length 

 1:2*48 to 1:3*00 (the latter in the young ones). Average propor- 

 tions in eight adult specimens : of fin-length (from insertion) to length 

 of dorsal side of mantle, abotit 1 : 2*55 ; breadth of fin to length of 

 mantle, average, 1 : 1*90; length of head (dorsal edge of mantle to 

 base of arms) to mantle-length, average, 1 : 7*15. 



The head is large, well-rounded ; the exposed portion is shorter 

 than broad, its breadth about equals that of the body, in ordinary 

 contraction ; its sides, in the region of the eyes, are somewhat 

 swollen; the under surface is flattened, and has a deep excavation 

 in front, semi-circular, or rather semi-elliptical, in outline, to receive 

 the dorsal half of the siphon-tube, which fits into it closely. 



The sides of the head, back of the eyes, have a rather prominent, 

 transverse ridge, back of which the head suddenly narrows, to the 

 neck. The transverse ridges curve backward slightly and meet on 

 the dorsal side of the head, where they are less prominent. Three 

 thin, lamelliform, erect folds of the skin extend backward from the 

 transverse ridge, on each side of the head ; of these the middle or 

 lateral one is about in line with the lower eye-lid ; the upper one is, 

 at its origin, about midway between the latter and the median dorsal 

 line, but its posterior edge bends downward and joins that of the one 

 below; the lowest of the three is shorter and curves upward, and 

 finally joins the middle one, at its posterior edge. These folds form, 

 therefore, in connection with the transverse ridge, two well-defined 

 lateral areas or facets, of delicate and probably very sensitive integu- 

 ment, placed just in front of the mantle-opening, on each side, where 

 they must be bathed by the inflowing currents of water. It seems 

 probable to me, therefore, that they are the seat of a special sense, 

 analogous to, if not identical with, that of smell. They are, also, 

 closely connected with the organs of hearing, and may be of some 

 service in concentrating soimd- vibrations. A small jDore is situated 

 within the lower facet. 



The pupils are round and the eyes are large, though the opening 

 between the lids is usually rather small, especially in alcoholic speci- 

 mens. In these the aperture is usually contracted to a small obliquely 

 transverse, irregular-triangular form, or even to a narrow oblique slit; 



