128 PROFESSOR STRUTHERS. 



distance ; best marked on the 9th, 10th, and 11th, where it is as 

 broad as the little finger.] 



In Megaptera the course of the beam is the same as in B. 

 musculus on both surfaces, but is rendered much less distinct 

 by the greater general thickness of the bone on each side of it. 

 On the 2nd and 3rd ribs the beam has already reached to about 

 the middle, and there is scarcely any of the grooving to its 

 inner side which is so strongly marked in B. musculus on the 

 2nd and 3rd, and even from the 4th to the 7th above their middle. 

 A trace of the sub-costal groove is seen from the 3rd to the 

 10th, from 6 to 12 inches in length, a little above the junction 

 of the lower and middle thirds of the bone, somewhat higher up 

 on the 10th. Where it is sharp-edged it has the breadth of a 

 goose-quill. 



Curvatures of the Ribs. — Although these ribs are more 

 simple than in most mammals, the four curvatures are seen and 

 differ in Megaptera and B. musculus — (1) The curvature of the 

 axis, enclosing the chest, is seen in the second column of the 

 table. Except on the 1st rib, in which it is 1 inch less (li- 

 on the left side), the depth of the curve is seen to be greater 

 in Megaptera than in B. musculus along the whole series, thus 

 giving Megaptera a wider thoracic cavity. It increases to the 

 longest rib in both. The greater curvature in Megaptera is 

 most striking on its last two ribs. 



Of the minor curvatures in (2) that of the borders, giving the 

 sigmoid form, the differences are not very marked. On the 

 outer border the concavity below the angle is seen in B. mus- 

 culus from the 1st to the 11th, owing to the greater prominence 

 of the angle ; in Megaptera on the 2nd and 3rd, and again on 

 the last four, but not much. On the lower third, the concavity 

 on the outer border, with convexity on the inner, is seen in 

 Megaptera from the 2nd to the 6th ; in B. musculus only on 

 the last four ; on the 13th, exceptionally well, the shaft of this 

 rib presenting a decidedly sigmoid form. These curvatures are 

 greatly exaggerated on the last rib of the Megaptera, and 

 usually on the last rib of B. musculus, as noted below. 



(3) Curvature on the Surfaces. — This is much more marked 

 in Megaptera, both above and below. In Megaptera the upper 



