368 MANUAL OF ZOOLOGY SECT. 



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 calcined 

 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, and two pairs of compressed neural plates (one 

 placed opposite the centrum, the other or . intercalary 

 cartilage, opposite the interval between adjoining centra), 

 which together form the sides of the arch, together with 

 usually two nodules the representatives of neural spines 

 which form the keystones. The transverse processes 

 are very short : connected with each of them is a cartilagi- 

 nous rudimentary rib about half an inch in length in a 

 specimen of average size. 



The cranium (Fig. 198) 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 elements 

 (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 (plf. cp.) 

 thin rounded cartilaginous sacs opening widely below the 

 cavities of the two capsules being separated from one another 

 by a thin septum. The part of the roof of the cranial cavity 

 behind and between the olfactory capsules is formed, not of 

 cartilage, but of a tough fibrous membrane, and the space 

 thus filled in is termed the anterior fontanelle : in contact with 

 the lower surface of the membrane is the pineal body, to be 

 afterwards mentioned in the account of the brain. Each 

 side-wall of this part of the skull presents a deep concavity 

 the orbit over which is a ridge-like prominence, the supra- 

 orbital crest, terminating anteriorly and posteriorly in obtuse 



