ANATOMY OF MEGAPTERA LONGIMANA. 129 



part of the rib is much bent forwards at the angle, while the 

 whole shaft is bent with the concavity backwards, giving a 

 marked sigmoid form when the rib is seen edgeways. In B. 

 musculus there is, above, only a gentle bend forwards on the 

 upper J, without any rapid bend at the angle; and below, 

 a well-marked bend backwards on the lower J of the shaft, 

 giving the sigmoid form. When the 6th ribs, the longest, 

 of Megaptera and B. musculus are laid together on the floor, rest- 

 ing on their outer border, the upper end rises, in Megaptera 10 

 inches, in B. musculus 6 inches ; and the lower half of the shaft 

 forms an arch 2J inches deep in Megaptera, 1J deep in B. mus- 

 culus. The rise at the upper end in Megaptera begins about 

 15 inches from the articular end, but much the greater part of 

 it takes place at the angle, about the middle of that distance. 

 The rise in B. musculus is by a gradual sweep along the upper J 

 of the bone, not marked at any particular place. If the external 

 neck be now made to lie level on the floor, the whole shaft in 

 Megaptera rises as an arch 13 inches high at the middle ; that 

 of B. musculus as an arch 5 inches high, and along its lower f 

 only, highest at J from the lower end. When these two ribs 

 are turned on their outer surface, and the external neck laid level 

 on the floor, the lower end rises from the floor, in the Megap- 

 tera rib 37 inches, in the B. musculus rib only 9 inches. The 

 differences are also well seen when the two ribs are made to 

 stand together vertically. These curves of the surface are much 

 better marked on the anterior half of the series (except the 1st) 

 than on the posterior half. 



(4) Torsion. The torsion or twist of the plane is seen below 

 on the lower half of the shaft ; above, on the upper part at the 

 angle, and specially at the articular end. The lower torsion is 

 very evident in B. musculus, the inner margin on the lower half 

 of the shaft twisted forwards, so that the surface which is 

 anterior above now looks outwards. In Megaptera, the greater 

 thickness of the shaft renders this torsion less obvious, but 

 it is greater than in B. musculus. The upper torsion is 

 much greater in Megaptera than in B. musculus. Along 

 with the upper bend forwards on the surface, the plane is 

 at the same time twisted, the inner (now lower) border back- 



