578 STRUCTURE OF VERTEBRATES 



The skulls of tetrapods can be reduced to four, or perhaps to 

 three, main types. In the earliest forms, the Stegocephalia and 

 reptihan Cotylosauria, the skull was completely roofed in and 

 covered by membrane bones, the only necessary gaps that were 

 left being the nares, the orbits, and the pineal foramen. The 



roofing bones fill up the fontanelles, 

 and lie close down on the nasal cap- 

 sule and on the membrane bones of 

 the upper jaw. Behind, however, they 

 are attached to the auditory capsule 

 only by a paroccipital process from 

 the opisthotic. There is thus a large 

 temporal space between the cartilage 

 bones, which make the side wall of 

 the cranium, and the lateral dermal 

 bones. This space houses the muscles 

 which work the lower jaw. Posteriorly 

 it has an opening, the post-temporal 

 fossa. This type of skull is called 

 anapsid, and is shown diagrammatic- 

 ally in Fig. 449 A. 



The only living tetrapods with a 

 skull of this type are the turtles and 

 tortoises (Fig. 450). It will be seen 

 that a number of bones present in the 

 primitive skull have been lost, and it 

 is possible, though now generally 

 considered improbable, that the com- 

 pletely roofed condition has been 



Fig. 451. — The skull of Capito- 

 saurus nasutus, one of the 

 Stegocephalia. — From Rey- 

 nolds, after von Zittel. 



I, Premaxilla ; 2, nasal ; 3, maxilla ; 4, 

 anterior nares ; 5, frontal ; 6, pre- 

 frontal {or lacrimal) ; 7, lacrimal 

 (or adlacrimal) ; 8, jugal ; 9, orbit ; 

 10, parietal ; 11, postfrontal ; 12, post- 

 orbital ; 13, interparietal foramen ; 

 14, supra temporal ; 15, squamosal ; 

 16, quadratojugal ; 17, quadrate ; 

 18, tabular ; 19, postparietal ; 20, 

 exoccipital ; 21, foramen magnum. 



The sheet of membrane bones which 

 forms the roof of this skull is a special 

 development of the armour of bony scales 

 which is found on other parts of the body 



of Stegocephali. It is not only the roof of i m • j \ ± u 



the cranium, but stretches over the space SeCOndanly aCqUireCl. A StegOCepIia- 

 between the cranium and the upper jaw in- 1 • t^* 



(palato-pterygo-quadrate bar). In most jian SKUll IS ShOWU lU tig. 4^1. 

 other bony skulls, gaps (the fossae) appear . , . 



between the bones of this dermal sheet. ThcrC IS SOmC disagreement aS tO 



the evolution of the reptilian skull 

 from that of the cotylosaurs, but it is at least certain that two 

 lines have been most successful. In one of these, the synapsid type, 

 the bony arcade covering the jaw muscles has been pierced by a 

 single lower temporal vacuity, which is below the postorbital and 

 postfrontal; it is, in fact, between the dermal bones of the cranium 

 and those of the upper jaw (Fig. 449 B). The reptiles which show 

 this condition, the Therapsida, are all extinct, but they left behind 

 them descendants in the mammals. In these, however, considerable 



