XX] ' VERTEBRAL COLUMN 475 



there are articulated a pair of neural arch-pieces or basidorsals 

 meeting in a median neural spine, and constituting a neural arch 

 enclosing the spinal cord and a pair of ventral arch pieces or basi- 

 ventrals uniting in the middle line in the tail region to form 

 a median haemal spine and so constituting a haemal arch 

 enclosing the caudal artery and vein, but in the trunk region the 

 haemal arch pieces do not meet : their outer ends become movably 

 articulated with the basal pieces and constitute ribs. A centrum 

 with its attached arches is called a vertebra. The ribs of Tele- 

 ostei do not correspond to the ribs of Chondrichthyes, for whereas 

 the ribs of that group of fish project into the midst of the myotomes, 

 the ribs of Teleostei curve down close to the peritoneal lining of the 

 splanchnocoel. In many Teleostei however there are curved bones 

 termed epipleurals which spring from the basal segment of the 

 haemal arch or basiventral and these bones correspond in 

 position with the ribs of Elasmobranch fish and are probably 

 homologous with them. 



There are no intercalates, either dorsal or ventral, in Teleostei. 



The centra are formed by the ossification of circular bands of con- 

 nective tissue outside the notochordal sheath which connect the neural 

 arch derived from one myotome with the haemal arch derived from 

 the one in front of it, for the myotomes slope downwards and back- 

 wards. In this -way the fully formed centrum comes to lie athwart 

 theTSeptum dividing two myotomes, and so provides attachment for 

 the ends of the muscular fibres derived from both myotomes (Fig. 232). 

 In many cases a cross section of the young vertebra shows us carti- 

 laginous neural and haemal arches projecting into a mass of bone 

 derived from the secondary notochordal sheath. The primary sheath 

 gives rise to neither bone nor cartilage but remains fibrous. 



Turning now to the skull we find that the cranium consists of 

 a broad ethmoid region in front and a broad occipital region 

 behind, joined by a narrow sphenoid region between the eyes 

 where its cavity is completely obliterated and it is reduced to a 

 vertical plate of bone or cartilage termed the interorbital septum 

 as in Holocephali. The cranium is at first entirely formed of 

 cartilage as in Chrondrichthyes, and, as in that group, there is a 

 membranous hole in the roof called the anterior fontanelle : but 

 the cartilage becomes covered with bone which partially or entirely 

 replaces it. In the posterior aspect of the skull there is the foramen 

 magnum or large hole through which the spinal cord passes to join 

 the brain. Around this are four bones, a supra-occipital with 

 a great median crest above, an exoccipitalat each end, and below 



