294 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [84] 



whether the mantle was in a contracted or expanded state when the 

 animal died. Caudal fin transversely rhomboidal, 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 margins are some- 

 what 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 (the latter in the young 

 ones). Average proportions, in eight adult specimens, of fin-length 

 (from insertion) to length of dorsal side of mantle, about 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 j 

 its sides, in the region of the eyes, are somewhat swollen ; the under 

 surface is flattened, and has a deep, nearly smooth excavation, semicir- 

 cular, or rather semielliptical, 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, lamelli- 

 form, 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 eyelid ; 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 

 evidently very sensitive integument, placed just in front of the mantle- 

 opening, on each side, where they must be bathed by the inflowing cur- 

 rents of water. A pair of large special nerves extends directly from the 

 cephalic ganglion to these organs. 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 the crests may be of some service in concentrating 

 sound vibrations. A small auditory pore is situated within the lower 

 facet. 



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

 tween the lids is usually rather small, especially in alcoholic specimens. 

 In these the aperture is usually contracted to a small, obliquely -trans- 

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

 more open, the aperture is still usually somewhat angular; the anterior 

 sinus is narrow and extends downward and forward. 



The eyelids form, when nearly expanded, an irregular oval, the longest 



