THE SKULL IN BIRDS. 



243 



able whole in most birds, but the sutures remain disting'uish- 

 able longer in the ChenomorphcB and Bphenisco'nior.phcB j and 

 especially in the Hatitm, 



All the constituents of the occipital and parietal segments 

 of the skull are represented by distinct bones, but the frontal 

 segment varies a good deal in this respect. The basisphenoid 

 has a long rostrum, which represents part of the parasphenoid 

 of the Ichthyopsida. Large frontal bones always exist, but 

 the pre-sphenoidal and orbito-sphenoidal regions are not so 

 regularly ossified. 



The ethmoid is ossified and frequently appears upon the sur- 

 face of the skull, between the nasal and the frontal bones ; and 

 the internasal septum, in front of the ethmoid, may present 

 very various degrees of ossification. Very frequently, the in- 

 terspace between the ethmoidal and the internasal ossifications 

 is simply membranous in the adult, and the beak is held to the 

 skull only by the ascending processes of the premaxillary 

 bones, and by the nasal bones, which are thin and flexible. 

 By this means a sort of elastic joint is established, conferring 

 upon the beak a certain range of vertical motion. In the Par- 

 rots, and some other birds, this joint is converted into a true 



MA 



so. 



BS 



SO 



Fig. 83. — A lon^tudinal and vertical section of the posterior half of the skull of an Ostrich 

 P., the pituitary fossa ; asc, j9sc., anterior and posterior vertical semicircular canals ol 

 the ear. 



articulation, and the range of motion of the upper beak be^ 

 comes very extensive. 



The periotic capsule is completely ossified, and, as in other 



