9 6 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



The bones are lighter than those of reptiles and are often pneumatic, that is, 

 are penetrated with canals connected with the respiratory system. The brain 

 cavity is larger than in reptiles; sutures between the bones largely disappear in the 

 adult, and the single occipital condole (mostly basioccipital) is on the floor of the 

 skull so that the axis of the skull is at right angles to that of the body. There is 

 only a single temporal fossa, bounded laterally by an arcade of jugal and quad- 

 ratojugal, connecting quadrate and maxillary. There is a preorbital vacuity; 

 and the nares may have the posterior margin rounded (holorhinal) or slit-like 

 (schizorhinal) . The premaxillaries are fused and sclerotic bones are common. 



A peculiarity of the ventral surface is the union of the anterior part of the 



FIG. 99. Earlier and later stages of skull of bird (Tinnunculus) after Suschkin. a/, 

 alisphenoid cartilage; ai, foramen for internal ophthalmic artery; b, basal plate; bpt, 

 basipterygoid; ec, external semicircular canal; hm, 'hyomandibular;' iorb, interorbital 

 plate ; it, intertrabecula; me, middle concha of nose; ov, occipital vertebrae; pc, posterior 

 semicircular canal; sorb, supraorbital; sir, supra trabecula; tr, trabecula. 



parasphenoid to the basisphenoid to form a 'rostrum sphenoidale' which projects 

 forward in the middle line. The rest of the parasphenoid forms a 'basitemporal 

 plate' below the basisphenoid and basioccipital. Dorsal to the rostrum is a 

 small presphenoid (sometimes lacking in the adult) to which the orbitosphenoids are 

 attached as alae, while the alisphenoids become similar wings to the basisphenoid. 

 Ectethmoids are connected with the mesethmoid; they are sometimes large, appear- 

 ing ('prefrontals') on the top of the skull. Epi- and ectopterygoids are lacking. 

 The pterygoids, here membrane bones, extend from the quadrates to the palatines, 

 and the two either slide along the rostrum or the vomers intervene. This, together 

 with the hinging of the front part of the skull upon the rest, forms a mechanism by 

 which the upper jaw is raised when the mouth is opened, the temporal arcade aiding 

 in the motion. The vomers may be paired; usually they form a thin vertical plate 

 between the anterior ends of the pterygoids; occasionally they disappear. The 

 choanae are between the palatines and vomers. Some birds have an *os uncin- 



