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. 8 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. 8 (his t, 

 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 Struthionidce, at the part where the post-frontal is 

 marked (at/") 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 harmonise 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. s) 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. 8 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. 8 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; 



p 2 



