252 MR. R. COLLETT ON BALH/NOPTERA BOREALIS. — [Feb. 2, 
VI. Flippers and other External Characters. 
The Flippers are relatively shorter than in any of the other species, 
somewhat slender and pointed. Their length, measured from the 
axilla, is about one eleventh of the total length, occasionally a little over 
and sometimes under this measurement. In form they do not differ 
materially from those of the other species. A little beyond the 
middle they present on the ulnar side a slight extension, an indication 
of the commencement of the manus. At this point the flippers have 
their greatest breadth, which is, compared to their length (reckoned 
to the axilla), as 1 to 3°5 or 3-6 (or to caput humeri as 1 to 4°7). 
Thus in its small flippers B. borealis differs essentially from B. stb- 
baldi, which it otherwise resembles in so many points, and approaches 
nearer to B. musculus. Their colour, as previously mentioned, is 
essentially the same on the outer and inner sides, and in this respect 
they differ from all the other species, in which the inner side is more 
or less white. 
The Dorsal Fin is situated far forward and is high. The front 
margin is convex, with a slightly curved apex ; its hinder margin 
is deeply concave. In form and position it is quite unlike that of 
B. sibbaldi and B. musculus, and is most like that of B. rostrata, 
but it lies further forward than in any of them. The following 
are the proportions between the total length of the body and the 
distance from the end of the snout to the beginning of the dorsal fin 
in five of the specimens examined by me :— 
No. 1.. As 1 to 0°65. 
No. 3.. As 1 to 0°68. 
No. 4.. As 1 to 0°65. 
No. 5 . As 1 to 0°65. 
No. 6.. As 1 to 061. 
The dorsal fin is thus constantly placed anterior to the com- 
mencement of the last third of the body, whilst that of B. rostrata 
(according to Sars) is placed at the beginning of this third; it is 
even further back in B. musculus, and furthest back in B. sibbaldi. 
Its height and size is on the whole considerable, especially in 
comparison with the length of the flippers. The height is in fact 
more than 13 times the greatest breadth of the flippers, and is to 
their length as 1 to barely 24. The dorsal fin is possibly higher in 
the male than in the female, although the difference cannot be 
great. 
The length of the anterior margin is about the same as that of 
the base. 
The Flukes, in a couple of specimens examined by me, were, com- 
pared to the total length of the body, as 1 to 4:2. 
Thus they were relatively somewhat smaller than in B. rostrata 
(3°4), but larger than in B. musculus (4°1-5:1). 
Their colour is, as in B. sibbaldi, about the same on the upper 
and lower sides. 
The Anus is situated directly under the apex and posterior margin 
