58 



LECTURE III. 



members of the class. So far as the observations of M. Agassiz have 

 extended, not one of the fossil fishes hitherto discovered in the Silurian 

 and Devonian rocks, the most ancient in which remains of that class 

 have been found, manifest a vertebral centrum ; and not many have 

 shown neural and hasmal arches and spines.* 



As a rule we find that the existing bony fishes have well ossified 

 vertebras, but retain a greater proportion, than in higher classes, of 

 the primitive gelatinous basis, which fills up the deep concavity of 

 each articular end of the centrum {fig. 14. c). Only in the sala- 

 mandroid Lepidosteus, with its lung-like air-bladder, does ossification 

 encroach upon these cavities, so as to render the anterior end of the 

 centrum convex, the posterior end concave {fig. 15.), and thus unite 



Scams. Lc/Jidosleus. 



the vertebrae together by ball and socket joints, (xxvi. p. 59.) 

 In the rest of the class, the vertebral bodies are connected together 

 by a strong elastic capsule, attached to the border of the base of each 

 terminal hollow cone, and enclosing the gelatinous fluid, which tensely 

 fills the biconcave space and renders the entire column light and 

 elastic. 



The vertebra of a bony fish consists essentially of a bicon- 

 cave body, of two neurapophyses {fig. 16. «) completing the canal 



Abdominal vertebra;, Pike (Esoi:). 



Abilomiiial vcrtobrse, Mugil. 



of the spinal chord, and usually supporting a spinous process {ns) ; 

 of two parapophyses (p) usually projecting from the lower part of the 

 sides of the body, or bent down to form the canal for the aorta {fig. 20.) : 



* Agassiz, Poissons Fossiles du Systeme Devonian, 4to, p. xxvi. 



