ON THE VERTEBRATE SKELETON. 203 
stronger proof of no.s being the petromastoid than of its being the squamosal : 
and for the same reasons that the articulation of no. s with the exoccipital, and 
its coalescence with the petrosal, are more essential characters of the petro- 
mastoid than they are of the squamosal, so I regard the articular surface 
furnished by no.s to the tympanic bone to be homologous with the articular 
surface of the petromastoid for the tympanic in the ruminants, rodents 
and other mammals, and am compelled to dissent from Dr. Hallmann’s idea 
of its answering to the articular surface furnished by the squamosal to the 
mandible in mammals. In the ostrich a part of the articular cavity for the 
tympanic is excavated in the exoccipital, and would afford as good an argu- 
ment to prove that bone to be the squamosal as the one which Dr. Hallmann 
has deduced from the same character in favour of the petromastoid in the 
bird being ‘the squamosal. Dr. Hallmann cites the junction of no. s (his ¢, 
taf. i. fig. 5, op. cit.) with the post-frontal in a young cassowary as evidence 
of its squamous character. I have not met with this union in the young 
ostrich nor in the young emeu, in which latter bird there is a distinct post- 
frontal: the anterior inferior angle of the parietal descends and meets the 
alisphenoid in both these Struthionide, at the part where the post-frontal is 
marked (at f"’) in Dr. Hallmann’s figure above cited. The extremity of the 
mastoid process does, however, arch over the temporal fossa to join the post- 
frontal process in certain birds, as above mentioned ; but this junction, when 
we ascend in our pursuit of the homologies of the elements of the composite 
temporal bone of mammals, as it is safest to do, from fishes to reptiles, 
and from these to birds, forms a repetition of a very characteristic feature 
of the mastoid in the cold-blooded classes, and one that is quite intelligible 
when we rise to the appreciation of the higher relations of both mastoid and 
post-frontal as parapophyses of their respective vertebra. 
In every mammal the squamosal is applied to the cranial parietes, and at- 
tached by a peculiar suture called squamous; the outer surface of the bone 
exceeding the inner surface. In no bird is the mastoid so united to the sur- 
rounding bones, but joins them by harmoniz vertical to the surface, as the 
other true cranial bones are joined before they coalesce; and the outer very 
little, if at all, surpasses the inner surface, to which the petrosal is confluent. 
The petromastoid of the mammal resembles that of the bird in this respect. 
‘There is no difficulty in the ascensive survey in appreciating the special 
homology of no. s in the bird (fig. 23) with no. s in the crocodile (fig. 22) 
and in the fish (fig. 5); and Dr. Hallmann, retaining a firmer and more 
consistent view of their common characters than Cuvier, enunciates clearly 
this homology: but having persuaded himself that the ‘ mastoid’ of the bird 
was its ‘squamosal,’ he concludes that the bone which Cuvier had called mas- 
toid in the crocodile and fish must also be their squamosal. I believe Cuvier 
to have rightly determined the bone (no.8) in the cold-blooded classes to be 
the mastoid ; but he is not consistent with himself when he adopts a different 
conclusion with regard to no.s in the bird. The greater development of 
the bird’s brain, as compared with the crocodile’s, requires a greater expan- 
sion of the cranial part of the mastoid, just as the still greater development 
of the brain in mammals calls forth a peculiar expansion and application of 
the cranial end of the squamosal, involving a transference of the mandibular 
joint to that expanded end. 
‘Cuvier, in descending from mammals to the consideration of the homolo- 
gies of no. s in the bird, passed too abruptly to the comparison, lacking the 
instructive link furnished by the monotremes. It might have sufficed for 
the present report to have demonstrated the homology of no.s in the bird, 
ascensively, with Cuvier’s well-determined mastoids in fishes and reptiles; 
: PQ 
= a 
