526 
Proceedings of the Royal Society 
greater relative breadth of rostrata was due to both the outer and 
inner surfaces being generally more convex ; but the anterior part 
of the outer surface of rostrata possessed a concave area more dis- 
tinctly marked than in borealis. In rostrata the superior border 
between the two attachments of the petrosal was more horizontal 
than in borealis , and in the same specimen the posterior attachment 
of the petrosal was nearer the hinder end of the bone, which was 
more rounded in rostrata than in borealis. In rostrata the entrance 
to the tympanic cavity was relatively wider, with the rounded border 
relatively thicker; whilst the inferior border was less distinctly keeled 
than in borealis. In the adolescent B. sibbaldii the tympanic was 
broader in relation to the length than in borealis , and this greater 
relative breadth was still more strongly marked in the immature 
sibbaldii , the breadth indices in the two specimens being respect- 
ively 49 and 53. The greater breadth of sibbaldii was especially 
seen in the posterior third of the bone, which bulged considerably 
on the inner surface in sibbaldii , and was almost flat in borealis. 
This bulging caused a, broadish concave surface at the root of the 
posterior attachment of the petrous bone in sibbaldii , whilst the 
corresponding area in borealis was a narrow groove. In sibbaldii 
the entrance to the tympanic hollow was more sinuous, and the 
rounded border of the bone much thicker than in borealis. In 
sibbaldii the anterior and posterior ends were less attenuated than 
in borealis. Through these characters there was no difficulty in 
distinguishing between the tympanies of these species, and the 
character which most impressed me in making this comparison was 
the relative want of breadth of borealis. I have not a tympanic 
bone of B. musculus at hand with which to compare my s]3ecimen, 
but from my recollection of a specimen I saw in the Royal Museum, 
Brussels, in May of last year, I should say that in musculus the 
tympanic had a greater breadth than in borealis . 
Pectoral Limb. — The scapula was a triangular plate, measuring 
29 inches between its superior and inferior angles, and 17-J inches 
in its glenoido-vertebral diameter. The coracoid was f inch long, 
and 2J in its greatest breadth. The acromion was 6f inches 
long, and 4 in its greatest breadth. The humerus was only 11 
inches long, and 1 foot 2J inches in circumference at the middle of 
the shaft, the surfaces of which were somewhat flattened. The 
