130 PROFESSOR STRUTHERS. 



wards. This twist is in the opposite direction to that of the 

 lower half of the shaft, but to a less extent. The torsions thus 

 accompany the two bends on the surface. The effect of the 

 upper bend and torsion is to give the upper part of the rib a 

 more horizontal direction inwards, and to render the plane of 

 its surfaces vertical ; the effect of the lower torsion is to make 

 the lower half of the shaft face more outwards as it sweeps 

 obliquely back along the thoracic wall. These differences in 

 the bend of the surfaces and in the torsion, between Megaptera 

 and B. musculus, would seem to be adaptations in Megaptera 

 to a more abrupt change from the transverse direction of the 

 upper part to the oblique direction of the shaft. 



The special torsion at the articular end is a rapid twist of 

 its lower part backwards, affecting also the lower part of the 

 inner half of the external neck. The result is that the articular 

 end, where it meets the transverse process of the vertebra, is 

 directed dowaiwards and backwards, decidedly so in B. musculus, 

 moderately so in Megaptera. 



The Last Rib. — This rib in Megaptera has the very undu- 

 lating character usually seen in the last rib in B. musculus and 

 other finners. This is but an exaggeration of the curvatures of 

 the other ribs, with, if the rib is long enough, the addition of a 

 third curvature to the sigmoid form. On the posterior border, 

 after the slight concavity external to the articular end (there is 

 no angle), there is the great convexity occupying about the upper 

 § of the bone ; then a wide concavity, occupying about the 

 middle ^ of the bone. These with corresponding curvatures on 

 the anterior border, complete the sigmoid form (like the' human 

 clavicle, but not so much bent as it), and there is only that in 

 the 15th rib of this B. musculus, and in the last rib of my B. 

 borealis. But in this Megaptera a third curve is present on the 

 lower J of the bone, convexity behind. The hinder margin thus 

 presents two convexities and one concavity, the anterior margin 

 two concavities and a convexity. The 13th rib, with much less 

 of the upper two bends, shows the third of these bends more 

 typically, owing to the tapering of its posterior margin towards 

 the end. The want of this tapering on the last rib (breadth of 

 end 2^ inches, of end of 13th, IJ inches), with a slight bend 



