528 BALENID A. 
including the Grampus and Hyperoodon, in having the 
hinder extremity of the bone simple and not bilobed ; and 
some of them differed, also, in having the anterior outlet 
of the cavity partially enclosed by the extension of the 
outer plate around that end. 
With regard to the Cachalot (Physeter), I had not had- 
the opportunity of comparing the Felixstow fossils, when 
I first gave an account of them, with the tympanic bone 
in that genus, which I then knew only by the figures given 
by Camper* in his characteristic but sketchy style. 
Cuvier, who founds his notice of the tympanic bones of 
the Cachalot on the same figures, states that they most 
resemble those of the Delphinide; but are less elongated 
and less bilobed posteriorly. The figures show still more 
clearly that the tympanic cavity is continued freely forward 
out of the anterior end of the bone, and terminates by a 
relatively wider outlet than in the Delphinida.t 
The idea thus given of the form of the tympanic bone 
of the Cachalot, being, as I have since had the opportunity 
of satisfying myself, in the main correct, the comparison of 
most of the Cetotolites becomes limited to the true whales 
(Balenida), in the few known species of which the dis- 
tinctive characters of the tympanic bones are afforded by 
their relative size and the shape of their inferior surface. 
In Balenoptera the tympanic bones, according to Cuvier, 
are very small in proportion to the head, and are equally 
convex at their inferior surface. 
* © Anatomie des Cetacés,’ Pls. xxill. xxv. 
++ ‘Ossemens Fossiles,’ 4to., v. pt. i. p. 376. 
+ Cuvier, (Legons d’Anatomie Comparée, ed. 1799, vol. ii. p. 492,) says, 
b) 
generally :—‘ L’extremité antérieure de la caisse est toute ouverte :” which cha- 
racter, M. Adrien Camper thinks he meant to apply to the Cachalot more par- 
ticularly. I shall combine the description of the petro-tympanic bone of the Ca- 
chalot, which I have recently received, with that of the Cetotolite most resembling 
it. 
