96 COMPARATIVE VERTEBRATES OF MORPHOLOGY. 
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 condyle (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 
Fic. 99.—Earlier and later stages of skull of bird (Tinnunculus) after Suschkin. al, 
alisphenoid cartilage; ai, foramen for internal ophthalmic artery; b, basal plate; bt, 
basipterygoid; ec, external semicircular canal; hm, ‘hyomandibular;’ iorb, interorbital 
plate; :t, intertrabecula; mc, middle concha of nose; ov, occipital vertebre; pc, posterior 
semicircular canal; sorb, supraorbital; str, supratrabecula; ér, 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 ale, 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 
choanz are between the palatines and vomers. Some birds have an ‘os uncin- 
