408 



COMPARATIVE ANATOMY. 



Fig. 247. Skull of a 

 Chelonian seen from 

 behind. 1 Basi-occipital. 

 2 Exoccipital. 3 Supra- 

 occipital. 5 Basi-sphe- 

 noid. 15 Prootic. 



17 Quadrate. 



visible. The greater development of the cranial capsule in Birds is 

 the cause of its parts being more distinctly visible than they are in 

 Reptiles. 



In the occipital region we can make out the four bones which 

 were present already in Fishes. Of these, the basi-occipital and the 

 exoccipitals take part in the formation of a single occipital condyle. 

 The relation that the bones have to the foramen magnum varies a 

 good deal. In the Chelonii the supra -occipital is continued into a 



large crest. As in the Amphibia there is a 

 fenestra ovalis in the osseous auditory cap- 

 sule ; and in addition to it there is a fenestra 

 rotunda-, which is closed by membrane. In all 

 Birds and Reptiles the petrosal (prootic) lies 

 in front of the exoccipital; its anterior edge 

 is distinguished by the foramen for the third 

 branch of the trigeminal. Another bone (opis- 

 thotic) unites with the prootic to form the 

 boundary of the hinder portion of the fenestra 

 ovalis ; but it does not remain separate in any 

 but the Chelonii, as in other forms it fuses 

 with the exoccipital. In addition to these, 

 there are several, and in Birds numerous, ossifications which are 

 independent for a short time ; these cannot be definitely regarded 

 as similar to any distinct cranial bones in the rest of the Vertebrata. 

 In Birds all the constituents of the auditory capsule fuse with the 

 neighbouring bones, as well as with one another. 



In the Ophidii the squamosal (Fig. 249, G Sq) is a projecting 

 bone, which carries the quadrate. In other Reptiles, and in Birds it 

 has the same position, but is more embedded between the auditory 

 capsule, parietals, and post-frontal, and is partly set in the roof of 

 the tympanic cavity. 



The sphenoidal segment varies in size according to the extent of 

 the cranial cavity. A basi -sphenoid is generally present, as is also the 

 presphenoid, which is ordinarily small ; the parasphenoid is no longer 

 developed. Two bones, however, which are seen at the base of the 

 temporal region in Birds, and which fuse with one another (basi- 

 temporals), may be regarded as parts of a parasphenoid. In Birds 

 there is an alisphenoid, and also an orbito-sphenoid — in the Ratitaa, at 

 least — which represent the side-walls of the skull. The Crocodilini, 

 also, are provided with an alisphenoid. In most of the Saurii, 

 however, there is an interorbital membranous septum, in which 

 indications merely of this bone can be made out. 



In the Saurii (Lacerta, Varanus, Podinema) there is a bone which 

 extends from the parietal to the jitcrygoid (columella) (Fig. 248, 

 A co) ; in the Chelonii this is represented by a broad plate of bone, 

 which passes straight down from the parietal, and which takes a 

 share in the limitation of the cranial cavity; in Hie Ophidii there is 

 a similar process, which embraces the cranial cavity, and extends on 

 to the frontal. 



