2IO 



CLASS AVES. 



vertebral column. In Reptiles a similar feature occurs in the 

 Ornithosauria. The inferior temporal arcade, formed by the jugal 

 and quadratojugal (fig. 1103), and connecting the maxilla with the 

 quadrate, is invariably present, but the superior temporal arcade is 

 always wanting ; and there are never distinct postorbitals or post- 

 frontals. The complete inclusion of the parietals in the roof of the 

 brain-case prevents the formation of the posttemporal fossae, which 

 constitute such a characteristic feature in the skulls of the majority 

 of Reptiles ; and no Bird has a parietal foramen. The base of the 

 cranium is formed by the basioccipital and basisphenoid, from the 

 latter of which proceeds the rod-like sphenoidal rostrum, represent- 

 ing the anterior part of the parasphenoid ; while the posterior portion 



is fr 



pmx ^ 



<JJ 



Fig. 1103. — Left lateral view of the skull of the Fowl, de, Dentary portion of the mandible ; 

 ar, Articular portion of do. ; qu, Quadrate ; sq, Squamosal ; eo, Exoccipital ; so, Supraoccipital ; 

 pa, Parietal ; fr, Frontal; la, Lachrymal ; na, Nasal; vo, Vomer; pmx, Premaxilla ; mx, 

 Maxilla; ju, Jugal ; qj, Quadratojugal ; pt, Pterygoid ; pi, Palatine; is, Interorbital septum. 



of the latter persists in the basitemporal plate underlying the basi- 

 occipital and basisphenoid. There is always a preorbital (lachrymo- 

 nasal) vacuity between the nasal, lachrymal, and maxilla (the tri- 

 angular space immediately behind the nasals in fig. 1103), as in 

 many extinct Reptiles ; and the interorbital septum is always more 

 or less ossified. The narial aperture (fig. 1103) is lateral, and 

 nearly always placed a short distance in advance of the orbit near 

 the root of the beak. The greater portion of the latter is formed, 

 as in the Ornithosauria, by the premaxillse, which coalesce at a very 

 early period in the middle line, and thus form a triradiate bone, 

 giving off a median nasal and a pair of lateral maxillary processes. 

 The pterygoids (fig. 1103) never unite together in the middle line 

 to form a completely closed palate ; and neither those bones nor the 

 palatines ever develop inferior palatal plates to separate the narial 



