204 REPORT—1846. 
but since both Cuvier and Dr. Hallmann have elucidated their views of its 
homology by characters drawn from the mammalian class, I have endeavoured, 
and I trust satisfactorily, to meet their objections and to determine the true 
homology of the bone by other arguments drawn from modifications of the 
petromastoid in the same class. 
Pursuing therefore the comparison descensively, I proceed in the next place 
to consider the characters of the mastoid in the crocodile (figs. 19 and 22, s). 
Cuvier premises his determination of the bone in that reptile by citing the 
following as its characters in the mammalia :—“ La partie mastoidienne qui 
recouvre le rocher en arriére de l’écailleuse et de la caisse, mais qui se soude 
de si bonne heure a ce rocher que l’on paroient 4 peine a la reconnaitre 
comme distincte dans les plus jeunes fétus ot elle est quelquefois double*.” 
The squamosal he defines as a bone “qui devient de plus en plus étrangére 
au crane 4 mesure qu’on descend dans l’échelle des quadrupédes, en sorte 
que dans les ruminans elle est plutdt collée dessus qu'elle n’entre dans la 
composition de ses paroist.” If we pause to apply these characters to the de- 
termination of nos. s and 27 respectively in the bird, before proceeding to 
the crocodile, we shall see how far they sustain the conclusions I have ar- 
rived at, in opposition to the views of Cuvier and his followers, in reference 
to the true homologue of the mammalian squamosal in birds. With regard 
to the mastoid in the crocodile, Cuvier says, “ Le mastoidien des crocodiles 
proprement dits et des gavials a cela de particulier, qu'il s’avance latérale- 
ment jusqu’a s’unir au frontal postérieur, et a entourer avec lui et le pari- 
étal le trou de la face supérieure du crane qui communique avec la fosse 
temporale ; dans quelques caimans il s' unit méme 4 ces trois os pour couvrir 
entiérement cette fosse en dessus, et dans les tortues de mer, non-seulement 
ils font la méme chose, le temporale et le jugal venant aussi a s’unir au mas- 
toidien et au frontal postérieure, ils couvrent la fosse temporale, méme par 
dehors.” £ 
Doubtless the German anatomists who dissent from Cuvier’s determination 
of the bone s in the crocodile (fig. 22) have been influenced in some degree 
by the little conformity between the character above assigned to the mastoid 
in that reptile and the character Cuvier had previously assigned to the mas- 
toid in mammalia. The confluence of the mastoid with the petrosal, for 
example, is a modification peculiar to the warm-blooded vertebrates, whilst 
the relative position of the mastoid, above and external to the petrosal and 
behind the squamosal and tympanic, is a constant character in all vertebrates ; 
to which must be added, that in most mammals and all other vertebrates the 
mastoid affords an articular surface for the tympanic bone, and developes an 
outstanding (mastoid) process for the attachment of strong muscles moving 
the head upon the trunk. 
With regard to the relative position of the mastoid process to the cranial 
walls, its origin ascends as the expansion of the parietal diminishes with the 
decreasing size of the cerebrum: in mammals, the process, when present, 
extends from the lower border of the postero-lateral wall of the cranium: 
in birds it projects from near the middle of that wall, and nearer the upper 
surface in the flat-headed Dinornis: in the crocodile it has ascended to a 
level with the upper surface of the cranium, and forms the posterior angle of 
that surface. The paroccipital presents a similar progressive ascent, but later 
in the series traced descensively ; it does not gain the level of the mastoid 
until we arrive at the class of fishes. 
The mastoid, thus determined in the crocodile, is recognized with ease 
and certainty in chelonia, lacertia and ophidia. It is a distinct bone in all 
* Op. cit. t. v. pt. ii. p. 81. + Ib. p.81. { Ib. p. 84. 
