110 PROF. W. B. BENHAM OlS" THE [May 21, 



It is situated about 12 inches from the tip of the snout ; but as 

 the measurement was made after the removal of skin and blubber, 

 it is probable that the fissure containing the blowhole has shrunk 

 backwards a little ; for in Owen's 6-foot specimen the blowhole is 

 only 5 inches from the tip of the snout. 



The distance between the horns of the crescent is 2| inches ; the 

 inner horn terminates (1| inch) further forwards than the outer. 



The blowhole is the external opening of a " vestibule " into 

 which open the two narial canals, of which the right is much the 

 smaller. The anterior wall of the opening has a sharply marked 

 edge or lip (PI. VIII. fig. 2, a), especially well-marked towards the 

 left, where it overlaps the posterior lip or " opercular fold " (d). 

 The hinder limit of the blowhole is, however, quite ill-defined ; the 

 surface of the head slopes gradually downward and disappears 

 behind the above lip, forming a kind of flap or operculum. 



On a closer examination of this apertm^e, it is seen to be im- 

 perfectly divisible into two portions : a small portion on the right 

 (b) just above the median line, and a much longer curved slit (a) 

 forming the greater moiety of the cresceuu ; the separation is indi- 

 cated by an interruption in the lip, for over a short space (c) the 

 anterior limit of the blowhole slopes gradually forwards to pass 

 below the operculum (d). 



On pressing apart the lip and operculum, or by cutting across 

 the lip, the crescentic blowhole is found to open into a shallow, 

 but wide chamber — the " spiracular sac " as Von Baer termed it in 

 Dolphins, or the " vestibule " as I would term it here. This is 

 lined by a blaek-pigmented epithelium continuous with that 

 covering the head, and passing downwards into the narial canal 

 on the left side. 



The greater part of this vestibule is, in reality, the upper end 

 of the large left narial canal, its floor is highly convex owing to 

 the existence of a very prominent fleshy "valve" (PL VIII. fig. 3, E) 

 which is developed on the mesial wall of the canal, but which, as 

 it approaches the top of the head, assumes a transverse and nearly 

 horizontal position. This valve becomes less convex as it approaches 

 the surface of the head, with which it is continuous at the spot c, 

 where the above-mentioned interruption in the lip of the spiracle 

 occurs. This valve reduces the cavity of the vestibule to a horse- 

 shoe-shaped cleft, the limbs of which pass anteriorly and posteriorly 

 towards the right ; the bend lying towards the left, where the cleft 

 deepens suddenly to form the left narial canal. 



In the Delphinidse this " spiracular sac " or vestibule is described 

 by Murie [4] as possessing in its fioor a pair of smooth, convex, 

 obliquely transverse cushions ; the above-mentioned " valve '' 

 appears to correspond with the left of these, but I find no mention, 

 in either of the works consulted, of the continuance of this con- 

 vexity down into, and along the wall of, the narial canal itself. 

 Indeed, in these papers, more attention has been paid to the 

 muscular arrangements than to details as to the dispositions of the 

 sacs and canals. 



