ANATOMY OF MEGAPTERA LONGIMANA. 137 



sider the very different proportions of the anterior aperture of 

 thorax in these two species. This is seen if the first pair of ribs 

 are laid at the natural distances from each other above and 

 below. In Megaptera the vertical diameter of the ring (from 

 the top of the ribs to the anterior border of their lower end) is 

 32 inches, the greatest transverse diameter 28 inches ; in B. 

 musculus the vertical diameter is 28 inches, the greatest trans- 

 verse diameter 35. Thus in Megaptera the width is 4 inches 

 less than the height, while in B. musculus the width is 7 inches 

 greater than the height. The contrast is still greater on the 

 lower part of the ring, the greatest width being above the 

 middle. This difference appears to determine the form of the 

 sternum in the two species. The sternum of B. musculus could 

 not be fitted between the naturally-placed ribs of Megaptera ; 

 the outer ^ or ^ of the wing would have to lie upon the rib. 

 The narrowness of the space between the first pair of ribs in 

 Megaptera requires a short-winged sternum, and the more 

 oblique course of the lower end of the rib is a reason why the 

 hinder edge of the wing should slope to the posterior process 

 instead of passing horizontally inwards. So in B. musculus the 

 wide ring requires a wide-winged sternum, and the nearly hori- 

 zontal direction of the hinder edge of the wing is in adaptation 

 to the nearly horizontal direction of the anterior border of the 

 lower end of the rib. These different relations also influence 

 the position of the marks for the terminal costo-sternal articu- 

 lation, and require the posterior process of the sternum to be 

 longer in B. musculus. 



The reasons for most of the differences seen in the table of 

 measurements (Table VII.) of the sternum in the two species 

 are now evident. Its somewhat diamond-shape in Megaptera 

 is owing to the shortness of the wings, and to their sloping to 

 a short posterior process ; the form of a cross in the B. musculus, 

 to the long wings and the long posterior process. The greater 

 massiveness of the ribs in Megaptera requires the sternum to be 

 thicker in it than in B. musculus.^ 



In Megaptera the cervical process is very broad, and is in an 



1 The weight of the first rib is— in Megaptera, right 182 ounces, left 168 ; in 

 the B. musculus— right 110^ ounces, left 110. But the left rib in the B. mus- 

 culus has a broader sternal end than the right. 



M 



