xii PHYLUM CHORDATA 369 



The spinal column is distinguishable into two regions, 

 the region of the trunk and the region of the tail. In the 

 trunk region each vertebra (Fig. 209, A] consists of a centrum 

 (c}, neural arch (na), and transverse processes (tr.pr). 

 In the caudal region there are no transverse processes, but 

 inferior or hcemal arches (D, h. a} take their place. The 

 centra of all the vertebrae are deeply biconcave or amphi- 

 coslous, having deep conical concavities on their anterior 

 and posterior surfaces. Through the series of centra runs 

 the notochord, greatly constricted in the centrum itself, 

 dilated in the large spaces formed by the apposition of the 

 amphicoelous centra of adjoining vertebrae. The concave 

 anterior and posterior surfaces of the centra are covered 

 by a dense calcified layer, and eight radiating lamellae of 

 calcified material run longitudinally through the substance 

 of the centrum itself. Each neural arch consists of a pair 

 of rod-like neural processes, which form the sides, and two 

 pairs of compressed neural plates (one placed opposite the 

 centrum, the other or intercalary cartilage, opposite the 

 interval between adjoining centra), which form the roof of 

 the arch, together with usually two nodules the repre- 

 sentatives of neural spines which form the keystones. 

 The transverse processes are very short : connected with 

 each of them is a cartilaginous rudimentary rib about half 

 an inch in length. 



The cranium (Fig. 224) is a cartilaginous case, the wall 

 of which is continuous throughout, and not composed, like 

 the skulls of higher vertebrates, of a number of distinct ele- 

 ments (bones) fitting in together. At the anterior end is a 

 rostrum, consisting of three cartilaginous rods converging as 

 they extend forwards and meeting at their anterior ends. 

 At the sides of the base of this are the olfactory capsules 

 (olf. cp), thin rounded cartilaginous sacs opening widely 



