ON THE VERTEBRATE SKELETON. 201 



contribution of the articular surface for the tympanic bone," which surface 

 he regards as homologous with the glenoid cavity of the squamosal for the 

 lower jaw in mammals. 



Cuvier, whose homology of no. 8 he thus adopts, describes it in the bird 

 as being on the outer side of the parietal, advancing also to beneath the 

 frontals, occupying the region of the temporal fossa and giving origin to the 

 temporal muscle, and as forming the superior border of the tympanic cavity. 

 " The temporal fossa," adds Cuvier, " is in great part excavated in the tem- 

 poral bone, and is bounded behind by a special process which might be re- 

 garded as the analogue of the zygomatic did it not remain far removed from 

 the malar bone*." The annotators add, " that there are some species of bird 

 in which, nevertheless, such zygomatic process does approach very close to 

 the jugalf ." 



First, then, with regard to the character which appears to have most 

 weighed with Cuvier, from his twice citing it in the above brief definition 

 of no. 8, — the marks of the origin of the temporal muscle. To conclude that 

 the bone impressed by the so-called ' temporal fossa' in the skull of the bird, 

 is therefore the temporal bone, because such fossa impresses a bone called 

 ' temporal ' in the mammal, is an example of that fallacy which logicians call 

 arguing in a circle. The two propositions by no means reciprocally prove 

 each other. Suppose, for example, that the bone no. s in the bird had been 

 determined, by way of ascensive comparison from the fish (fig. 5) and cro- 

 codile (fig. 16), to be the homologue of the bone no. 8 in those animals, which 

 we will assume to have been rightly called ' mastoid ' by Cuvier, and that he 

 had arrived at the determination of no. 8 in the bird by this surer method, 

 than by the descent from placental mammals ; and supposing that, having thus 

 recognized no. s as the mastoid, the fossa and muscle with which it is im- 

 pressed in the bird had been called ' mastoidal ' instead of ' temporal '; then, 

 ascending to the mammalian cranium, Cuvier might with equal reason have 

 said that the bone 27, figs. 1 1 and 22, was the ' mastoid,' because it occupied the 

 region of the mastoidal fossa and gave origin to the mastoidal muscle. The 

 origins of muscles are not, however, sufficiently constant to be included amongst 

 the characters of connection or function determinative of special homologies. 

 The transference of the ' sterno-mastoideus ' from the true mastoid process 

 (Man, Carnivora and Rodentia) to the angle of the mandible (horse), and to 

 both this part and the second cervical vertebra (Ruminants), shows that the 

 attachments of a muscle must be determined after the recognition of the bone, 

 and not the homology of the bone by muscular attachments. With the very 

 case in question the uncertainty of the character is illustrated : in the skull 

 of the ostrich, for example (fig. 8), the temporal fossa is chiefly formed by the 

 conjoined portions of the parietal (7) and alisphenoid(e), which intervene be- 

 tween the mastoid (s) and the post-frontal, the mastoid forming not more of 

 the posterior part of the fossa than the post-frontal does of the anterior part. 

 Dr. Hallmann probably appreciated the unsoundness of the argument from 

 the muscular impression, since he does not cite it ; he repeats, however, the 

 character adduced by Cuvier, from the relation of no. 8 to the tympanic 

 cavity, or as Hallmann expresses it, the meatus auditorius (aussern Gehor 

 offnung), the value of which therefore I next proceed to consider. 



In the skull of the ostrich, with the tympanic bone and ear-drum in place, 

 the upper border of the meatus, as defined by the periphery of the membrana 

 tympani, is formed, not by no. 8, but by the tympanic anteriorly, and by the 

 paroccipital process (4) posteriorly. When the tympanic bone and mem- 

 brane are removed, then the descending process of no. 8 overarches the 

 * Le?ons d'Anat. Comp. ii. (1837), p. 580. f lb. p. 581. 



1846. p 



