168 PROFESSOR STRUTHERS. 



Premaxillary Bone. Besides the transverse convexity above 

 noted, the premaxillary in Megaptera is later in undergoing 

 the seeming twist of the surface. In B. musculus the pre- 

 maxillary, very concave along the distal half of the prenasal 

 space, becomes nearly horizontal at about a third of the distance 

 into the second quarter of the beak, and also nearly flat, except 

 that there is a shallow groove along its inner third (prolonged 

 from the concavity at the prenasal space) and a gentle con- 

 vexity on the outer third. 



In Megaptera the premaxillary on the distal half of the 

 prenasal space is scarcely concave, and at the end of the first 

 quarter of the beak the surface looks more inwards than 

 upwards, and does not become horizontal till near the middle 

 of the beak. 



This seeming twist of the premaxillary, however, is not in 

 reality a torsion of a flat bone, but is owing to the development 

 of a transverse plate, beginning on the distal half of the pre- 

 nasal space, which goes inwards and contracts that space. The 

 premaxillary thus attains a sharply triangular form in trans- 

 verse section. The internal or nasal surface is very concave in 

 Megaptera from the end of the prenasal space onwards ; in B. 

 musculus the cavity does not begin till near the middle of the 

 beak. In Megaptera the internal lamina is much more de- 

 veloped on the second quarter of the beak than onwards from 

 this, the approximation of the premaxillaries being accomplished 

 along the distal half by the more inward position and more 

 inward slope of the vertical plate of the bone. In B. musculus 

 the inward plate of the premaxillary appears to overhang to 

 about the same extent throughout. 



The width of the inter-premaxillary space along the distal 

 three-fourths of the beak is, at the beginning of each quarter, 

 respectively, in inches, in Megaptera, 8 J, 3, 2 \ ; in B. Musculus, 

 2J, 1J, If. On the distal quarter, in Megaptera it continues 

 to contract gradually to 1 J inch at the point ; in B. musculus 

 it at first widens to 2 inches, and then contracts, width at the 

 point about f inch. 



19. FORAMINA ON THE FACIAL SURFACE OF THE MAXILLARY. 

 These large foramina present differences in Megaptera and B. 

 musculus, but reliance cannot be placed on characters which 



