174 PROFESSOR STRUTIIERS. 



face, in B. miisculus the smooth part is marked off from the 

 rough part by a much more defined border than in Megaptera. 

 5. This smooth part is more bulged in Megaptera on its 

 posterior half, contributing to the increased breadth on the 

 posterior half of the bone. 6. The aperture is more constricted 

 in Megaptera at the division into anterior and posterior parts. 

 7. The anterior (Eustachian) division presents marked differences 

 in form. In B. musculus its anterior end bends upwards to a 

 subacute point, so that the lower boundary of the aperture is 

 nearly uniformly concave. In Megaptera the anterior end is 

 directed obliquely downwards and forwards, and is almost 

 square-shaped, so that the lower boundary is convex behind, and 

 then very concave in front. The width of the anterior division 

 of the opening is greater in Megaptera than in B. musculus ; 

 greatest width at its posterior half, owing to a bend up of the 

 wall here at the anterior junction with the petrosal, much more 

 marked than in B. musculus. The ridge which here forms the 

 exact upper boundar}^ of this division of the aperture is smooth 

 and concave in B. musculus. It is very thin and sharp in 

 Megaptera, and goes spirally forwards on the smooth surface 

 in front. 8. The width of the posterior (meatus externus) 

 part of the aperture is also greater than in B. musculus, at 

 its last half inch ^ to ^ inch in B. musculus, f to ^ inch in 

 Megaptera. 



Viewed from the outer side, two depressions are seen, dividing 

 the surface on its upper half into three lobes. The anterior de- 

 pression corresponds to the constriction between the Eustachian 

 and the meatus divisions of the aperture. The middle lobe is 

 continuous with the broadest part of the bone, and, upwards, 

 there is prolonged from it, at its back part, a flat lamella which 

 projects in front of the external meatus. The third or posterior 

 lobe does not extend so far on the under surface as the other 

 lobes, and is continued upwards to end in a nipple-shaped pro- 

 cess, which forms the projecting lower boundary of the meatus 

 externus. 9. In B. musculus the pre-meatus lamella is more 

 bent, sigmoid behind (concavity below the middle). 10. An 

 antero-posterior depression in front of the base of the lamella, 

 bounding the middle lobe above, is much better marked in B. 

 musculus, cutting the lamella off from the lobe, except behind. 



