282 THE ANATOMY OP VERTEBRATED ANIMALS. 



fewer of tlie ribs. The two last-named structures are very 

 distinct in the Coracomorphce, or Passerine Birds. 



The extent to which the keel of the lophosteon is de- 

 veloped in the carinate birds varies very much. In Strigops 

 it is rudimentai-y ; in birds of powerful flight, as well as in 

 those which use their wings for swimming, it is exceedingly 

 large. 



In the bird's skull (Fig. 82), the brain-case is more arched 

 and spacious, and is larger, in proportion to the face, than 

 in any Reptilia, with the exception of the Pterosauria. 

 There is a well-marked interorbital septum, but the extent 

 to which it is ossified varies greatly. As a general rule, 

 the superior temporal bar is incomplete, and there is no 

 distinct post-frontal bone. The inferior temporal bar, 

 formed by the jugal and quadrato-jugal, on the other hand, 

 is always complete. There are no long parotic processes, 

 nor any post-temporal fossae, the whole of each parietal 

 bone being, as it were, absorbed in the roof of the skull. 



The nasal apertures are almost always situated far back 

 near the base of the beak. In the dry skull (above Mx. 

 in Fig. 82), there is a lachrymo-nasal fossa, or interval 

 unoccupied by bone, between the nasal, lachrymal, and 

 maxillary bones, such as exists in some Teleosauria, Dino- 

 savria, and Pterosauria. 



The posterior nares lie between the palatines and the 

 vomer ; and the nasal passage is never separated from the 

 cavity of the mouth by the union of palatine i^lates of the 

 palatine or pterygoid bones. 



The Eustachian tubes generally traverse the basisphenoid, 

 and have a common aperture upon the middle of the under 

 surface of the skull. 



The bones of the brain-case, and most of those of the face, 

 very early become ankylosed together into an indistinguish- 

 able whole in most birds, but the sutures remain distinguish- 

 able longer in the Chenomorphce and Spheniscomorphce ; and 

 especially in the RatitcB. 



AH the constituents of the occipital and parietal seg- 

 ments of the skull are represented by distinct bones, but 



