The tympanic and periotic bones of _B. bairdii_ (Pls. 34-37) present a
number of characters by which they may be distinguished from those of _B.
arnuxii_. While of about the same size in both species, the two bones
when in the natural position, viewed from without, are nearly square
rather than triangular in outline in _B. bairdii_, the superior border of
the periotic being nearly parallel with the inferior border of the
tympanic, and the anterior lobe of the periotic being turned down nearly
at right angles with the rest of the bone. The periotic is shorter
anteriorly than the tympanic in _B. bairdii_, while the reverse is true
in _B. arnuxii_. In the former species the eustachian canal of the
tympanic is wider, the distance between the outer and inner lips being
greater. The involuted portion of the inner lip is shorter and
differently shaped. The groove between the postero-inferior lobes is
wider. The periotic beside having a much shorter anterior lobe than in
_B. arnuxii_ has also a smaller and smoother middle lobe, and the
internal auditory meatus is smaller and more oblique. The dimensions of
the bones in the Centerville beach skull, No. 49725, are as follows:
_Tympanic_: greatest length, 62 mm.; greatest breadth, 46; least breadth
of eustachian canal, 17; height at sigmoid process, 47. _Periotic:_
greatest length, 66; greatest breadth, 40; height at center of middle
lobe, 35; length from tip of anterior lobe to anterior margin of internal
meatus, 38.