XIII 



PHYLUM CHORDATA 



137 



ce-n.1 



haem 



calcified layer, and eight radiating lamellaB of bone (O) run longi- 

 tudinally through the substance of the centrum itself. The centra, 

 unlike those of the higher forms, are developed as chondrifications 

 of the sheath of the 

 notochord into which 

 cells of the skeletogen- 

 ous layer have migrated 

 (p. 66). Each neural 

 arch consists of a pair 

 of rod-like neural pro- 

 cesses, which form the 

 sides, and two pairs of 

 compressed neural plates 

 (one placed opposite the 

 centrum, the other or 

 intercalary cartilage, op- Flo . 767.-Chiioscyiiium, vertebra. A, end view of 



trunk vertebra, cent, centrum ; neur. neural plate 

 and process ; sp. neural spines ; r. ribs ; Ir. proc. 

 transverse processes. B, lateral view of the same. 

 hcem. haemal arch ; neur. neural arch. C, transverse 

 section of a centrum, showing radiating lamellse of 



bone. 



posite the interval be- 

 tween adjoining centra) 

 (Fig. 768), which form 

 the roof of the arch, 

 together with usually 

 two nodules the representatives of neural spines (sp.) which form 

 the keystones. The transverse processes are very short : connected 

 with each of them is a cartilaginous rudimentary rib (r.) about 

 half an inch in length. 



The cranium (Fig. 768) 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 in Scyllium 

 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.) 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 obscure processes termed respectively 

 the prce-orlntal and post-orbital processes. Below the orbit is a 

 longitudinal infra-orbital ridge. 



Behind the orbit is the auditory region of the skull a mass of 

 cartilage in which the parts of the membranous labyrinth of the 

 internal ear are embedded. On the upper surface of this posterior 



V 



