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 masto'fdienne qui 

 recouvre le rocher en arriere de l'ecailleuse et de la caisse, mais qui se soude 

 de si bonne heure a ce rocher que Ton paroient a peine a la reconnaitre 

 corame distincte dans les plus jeunes fetus ou elle est quelquefois double*." 

 The squamosal he defines as a bone " qui devient de plus en plus etrangere 

 au crane a mesure qu'on descend dans l'echelle des quadrupedes, en sorte 

 que dans les ruminans elle est plutot collee dessus qu'elle n'entre dans la 

 composition de ses paroisf." If we pause to apply these characters to the de- 

 termination of nos. s and <n 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 laterale- 

 ment jusqu'a s'unir au frontal posterieur, et a entourer avec lui et le pari- 

 etal le trou de la face superieure du crane qui communique avec la fosse 

 temporale ; dans quelques caimans il s'unit meme a ces trois os pour couvrir 

 entierement cette fosse en dessus, et dans les tortues de mer, non-seulement 

 ils font la meme chose, le temporale et le jugal venant aussi a s'unir au mas- 

 toidien et au frontal posterieure, ils couvrent la fosse temporale, meme par 

 dehor.-." % 



Doubtless the German anatomists who dissent from Cuvier's determination 

 of the bone 8 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 Dinomis : 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. t lb. p. 81. % lb. p. 84. 



