OSSEOUS SYSTEM OF AVES. 



43 



the basis of the tympanic and mandible ; that of the third arch 

 forms the stylo-hyal, rarely ossified in birds, and in connection 

 Avith it is developed the ^ stapes.' The product of the fourth is 

 homoloo-ous with a branchial arch in the fish: but further evi- 

 dence of such conformity with the segmental structure of the 

 trunk-skeleton as is discernible in the much modified anterior 

 termination of the body is given by the ossific centres established 

 in the primordial cartilages ; and by the special homologies, de- 

 terminable prior to confluence, of the bones developed therefrom, 

 with the skull-bones of the lower cold-blooded Vertebrates, which 

 retain their distinctness and depart less from the archetypal 

 arrangement. 



Although, as a general rule in the class Aves, the separate 

 cranial bones can be discerned only at an early period, yet in 

 those birds in which the power of flight is abrogated, the indica- 

 tions of the primitive centres of ossification endure longer ; and 

 in the species here selected for the illustration of the cranial 



26 



Side view of disarticulated cranial vertebra and sense-capsules. Ostrich. 



segments, the constituent bones of the skull, with the exception 

 of the basioccipital, i, the basi-pre-sphenoid, 5, 9, and the bones 

 2, 6, and 8, which coalesce with the petrosal, 16, have been sepa- 

 rated by maceration merely in the half-grown bird. 



The basioccipital, figs. 26, and 27, i, developes the major part 

 of the single articular condyle, and sends down a process, more 

 marked in the Struthious genera, and especially in Aptornis, than 

 in most other birds : in all respects this primitively distinct bone 

 retains the character of the centrum of its vertebra. 



