64 



LECTURE III. 



for by the characteristically well-developed body of the atlas in 

 fishes, since the atlas has a small centrum in crocodiles and birds, 

 where the odontoid process likewise exists. 



The number of vertebrae varies greatly in the different osseous 

 fishes : the Plectognathi {Diodon, Tetrodon,) have the fewest and 

 largest : the apodal fishes (Eels, Gymnotes,) have the most and 

 smallest, in j^roportion to their size. It is not often easy to deter- 

 mine the precise number, on account of the coalescence of some of 

 the vertebrte, or at least of their central elements, in particular parts 

 of the column. Instances of anchylosis of some of the anterior ver- 

 tebra, analogous to that noticed in the cartilaginous Sturgeons, 

 Chimaera^, Rhinobates, and some Sharks, occur also amongst the 

 osseous fishes, as in many Siluroid and Cyprinoid species ; in the 

 Loricaria and Fistularia : here is an example {Jig- 21.) of the four 

 singularly elongated anchylosed anterior abdominal ver- 

 tebra, in the Tobacco-pipe fish {Fistularia tabaccaria). 

 A coalescence of several vertebrae is more constant at the 

 opposite end of the column in osseous fishes, in order to 

 form the base of the caudal fin. The bodies at least of the 

 vertebrae situated here, at the part most remote from the 

 centre of life, do not emerge separately from the primitive 

 embryonic condition of the gelatinous 'chorda,' but are 

 continuously ossified to form a common, compressed, ver- 

 tically extended, and often bifurcated bony plate (^^. 18. 

 nh'), from which the neural and haemal arches and their 

 f\ i'l ^ spines radiate : from these elements alone can the number 

 of vertebrae of the caudal fin be estimated ; normal de- 

 velopment proceeding here in the peripheral elements, as 

 thi'oughout the vertebral column in Lepidosiren, whilst 

 it is arrested in the central parts of the vertebrae. In 

 the Sun-fish {^Orthagoriscus moki) it would seem as if a 

 row of rudimental vertebrae had been blended together at 

 right angles to the rest of the column, in order to support 

 the rays of the short, but very deep caudal fin, which ter- 

 minates the suddenly truncated body of this oddly shaped 

 {Fistularia). ggj^_ q^^, commou Pike affords a simple and intelligible 

 view of this modified base of the tail-fin : in the Eels, the Polypterus, 

 the Lepidosiren, the Trichiurus, and Pipe-fishes, the vertebra? always 

 remain distinct to the end of the tail. 



Cuvier, in the tables of the number of vertebrae in various species 



to the several parts of this comhined segment of cndo- and exo-skeleton are oppo- 

 site the left hand of the reader ; those applied to them in the present work are 

 placed opposite the right hand. 



