XIII 



PHYLUM CHORDATA 



143 



pulpy mass. The concave anterior and posterior surfaces of the 

 centra are covered by a dense calcined layer, and in Heiniscyllium 

 eight radiating lamellse of calcined tissue run longitudinally 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 skeletogenous layer have 

 migrated (p. 71). On the dorsal side of the row of centra the 

 spinal column is represented by the series of neural arches which 

 support the walls of the spinal canal. Owing to the presence of a 

 series of intercalary cartilages the neural arches appear to be twice 

 as numerous as the centra. Each neural arch consists on each side 

 of a process, the neural process, given off from the centrum, and 



trL 



ip.br.5 



FIG. 823. Hemiscy Ilium, lateral view of skull with visceral arches and anterior part of spinal 

 column ; the branchial rays are not represented. The skull and hyoid arch are somewhat 

 drawn downwards, so that the hyoid and first branchial arch are not exactly in their natural 

 relations ; cer. hy. ceratohyal ; ep. br. 1, ep. br. 5, first and fifth epibranchials ; ql. aperture 

 for glossopharyngeal nerve ; b. hy. basihyal ; hy. run. hyomandibular ; interc. intercalary 

 (interdorsal) plates ; mrk. Meckel's cartilage ; neur. neural processes ; olf. olfactory capsule ; 

 oc. foramen for oculomotor ; oph. 1, foramen for ophthalmic division of facial nerve ; 

 opk. 2, foramen for ophthalmic division of trigeminal ; opt. optic foramen ; pal. </. palato- 

 quadrate ; path, foramen for 4th nerve ; ph. br. 1 and ph. br. 5, first and fifth pharyni>- 

 liranchial ; sp. neural spines; tr. transverse processes and ribs; tri. foramen for 

 trigeminal nerve. 



of a small cartilage, the neural plate (basi-dorsal), which becomes 

 completely fused with the neural process in the adult. Between 

 successive neural plates, the width of each of which is only about 

 half the length of the centrum, are interposed a series of plates 

 of very similar shape, the interdorsal or interneural plates. Small 

 median cartilages, the neural spines, fit in between both neural 

 and interneural plates of opposite sides and form keystones com- 

 pleting the arches. 



The transverse processes are very short : connected with each of 

 them is a rudimentary cartilaginous rib (r.) about half an inch in 

 length. 



The cranium (Fig. 823) is a cartilaginous case, the wall of which 



