XII 



PHYLUM CHORDATA 



391 



radiating or concentric lamellae of calcified cartilage or 

 bone, or they may be completely calcified. They are 

 deeply amphiccelous, the remains of the notochord per- 

 sisting in the large spaces between the concave surfaces 

 of contiguous centra. In the rays the anterior part of the 

 spinal column becomes converted into a continuous solid 

 cartilaginous and bony mass the anterior vertebral plate 

 (Fig. 233, a. v. p). Two main regions only are distinguish- 



pt. orb 



FIG. 234. Lateral view of the skull of Heptanchus. mck, Meckel's cartilage; 

 pal. au, palato-quadrate; pt. orb, post-orbital process of the cranium, with 

 which the palato-quadrate articulates. (After Gegenbaur.) 



able in the spinal column the pre-caudal region and the 

 caudal, the latter being distinguished by the presence of in- 

 ferior or haemal arches. In the pre-caudal region short ribs 

 may be developed, but these are sometimes rudimentary or 

 entirely absent. 



The skull is an undivided mass of cartilage, hardened, in 

 many cases, by deposition of osseous matter, but not con- 

 taining any separate bony elements. In all, the jaws are 



