22 



VERTEBRATE SKELETON 



separate (p. 37), but there is more or less fusion of parts, though in 

 different ways, in most groups. It is stated that in some lower fishes 

 (as Holocephali) cranial and caudal elements of the same original 

 myotome unite to form a single vertebra, the cranial parts in front, 

 the caudals behind, the vertebra being thus intrasegmental. * In all 

 others (fig. 20) the caudal parts of one somite unite with the cranials 



CHfLIDOSAURUS 



CATURU5 CALLOPTERUS [URYCORMUS AMIA 



Fig. 20. — Scheme of vertebral elements in Stegocephal (Chelidosaurus) and Ganoid 

 vertebrae, c, centrum; cdh, caudihaemal; cdn, caudineural; crh, cranihcemal; cm, 

 cranineural; ha, hypocentrum arcale; hp, hypocentrum pleurale; /, intercentrum 

 (intercalare) ; n, neurapophysis; nc, notochord; p, pleurapophysis. 



of the next posterior to form the definitive vertebra which is thus 

 intermyotomic, a condition more advantageous for the action of the 

 trunk muscles. Details are given below. 



The anterior and posterior faces of the centra may have different 

 shapes in different groups. The most primitive has both faces 

 excavate — amphicoelous vertebrae (fig. 21, A). In some fishes and 

 many reptiles the centra are hollow in front and convex behind (fig. 

 21, B) the two vertebrae articulating by a ball and socket joint 



Fu; 



-Sagittal sections of (A) amphicoelous, (B) procoelous, (C) opisthocoelous, and 

 (D) amphiplatyan vertebrae; the left is anterior. 



(procoelous vertebrae). In opisthocoelous vertebrae these conditions 

 are reversed, the socket being on the posterior face (C). Most 

 Mammalian vertebras have the two faces fiat (amphyplatyan) an 

 intervertebral disc (meniscus) intervening between each two centra 

 (D). An exception should be noted in the cervical region of Artio- 

 dactyls where most of the vertebrae are opisthocoelous. 



