74 MORPHOLOGICAL TYPES 



It will be obvious from this table that the majority if not 

 all the cartilage-bones are ossifications in cartilage which 

 itself is represented in the dogfish. On the other hand, there 

 are no structures of any kind in the dogfish which have any 

 connexion with the membrane-bones. The membrane-bones 

 are of interest from two points of view. A number of them 

 bear teeth : premaxilla, maxilla (not in Gadus, however), pre- 

 vomer, dentary. Others enter into relations with the lateral- 

 line canal system, and these relations are of importance, for 

 owing to their constancy they enable homologies to be made 

 between bones in fish and in higher Vertebrates. The lateral- 

 line canal of the trunk runs forwards from the tail, and in so 

 doing it pierces the scales. On reaching the head it is pro- 

 tected by a few " lateral-line ossicles," and then passes through 

 the post-temporal to the pterotic and sphenotic. The supra- 

 orbital canal runs forwards over the eye through the frontal 

 and nasal, the infraorbital canal pierces the chain of bones 

 formed by the post-orbitals, infraorbitals, and lachrymal. 

 The hyomandibular canal runs down through the preopercular 

 to the dentary. 



Teeth. — The teeth are fundamentally similar to the denticles 

 or placoid scales of the dogfish, but instead of being scattered 

 all over the surface of the body, they are restricted to the 

 mouth. They are composed of a core of dentine containing 

 a pulp cavity, and are covered over with a cap of enamel. 

 The bones which bear teeth have been enumerated above. 



Nervous System. — The brain and spinal cord lie in the 

 long tubular cavity provided by the skull and neural arches of 

 the vertebrae. The spinal cord is essentially similar to that 

 of the dogfish, and calls for no special description. The spinal 

 nerves, each composed of a dorsal and a ventral root, emerge 

 between the neural arches. 



- In the brain, the medulla oblongata is not very different 

 from the spinal cord. The cerebellum is well developed and 

 projects downwards and forwards beneath the roof of the 

 midbrain forming the valvula, a structure which is peculiar 

 to bony fish. The roof of the midbrain is produced into optic 

 lobes. The floor of the forebrain projects downwards as the 



