108 PROFESSOR STRUTHERS. 



between the condyloid cups, as seen in these five specimens of 

 B. musculus, may be a matter of age, but in the atlas of a sixth 

 great finner (referred to, loc. cit, 1872, p. 15 and p. 45), larger 

 than any of the other five, the median groove is broad ; at the 

 narrowest, at an inch from the lower end, 1 J inch, at the middle 

 If, near the canal If. It may possibly become narrower with 

 age in Megaptera also. 



The inferior ends of the condyloid cups project more than 

 in B. musculus. This projection in the forward direction renders 

 the cup somewhat deeper than in B. musculus ; but the chief 

 difference is the greater downward projection, by which in 

 Megaptera the cups project below the level of the inferior 

 arch of the bone, leaving a wide and deep notch between 

 them. In the four mature specimens of B. musculus the rough 

 anterior arch is seen below the level of the cups, and the notch 

 between the slightly raised inferior ends of the cups is shallow, 

 J to | inch deep, and about 3 inches wide, but in the largest 

 atlas (the Wick specimen) considerably narrower. In the 

 Megaptera the notch is 1J inch deep, and, in width, 3J inches 

 below, 1J above. In the 50-feet-long B. musculus the distinc- 

 tion is much less marked, the notch 1 inch deep and wider 

 below than in Megaptera, and the cup projects below the level 

 of the inferior arch of the bone. But allowing for the 

 immaturity of both, as compared with the four first-mentioned 

 specimens of B. musculus, it appears that the lower ends of 

 the cups project considerably more in Megaptera than in B. 

 musculus, giving both a deeper cavity and a greater projection 

 downwards. 



Posterior Aspect of the Atlas. The chief characters on this 

 aspect of the atlas are (a) the presence of a mesial articular 

 surface, dividing into three parts what forms one great horse- 

 shoe articular surface in other finners, and (b) the form of the 

 ligamentous area. 



(a) Mesial Articular Surface. The position and characters 

 of this surface are seen in fig. 17. It occupies the whole 

 height and breadth of the inferior arch of the bone ; in form 

 resembling the upper half of a blunt oval, height 2J inches, 

 width 3 inches, vertically convex on its upper half, on its lower 

 half a little concave. It is bounded below by the triangular 



