ANATOMY OF VERTEBRATES. 41 



xmder part of the centrum, as, e. g. the two which articuhate with 

 the basioccipltal in the Arapaima gigas. As the centrum of the 

 athis retains its normal relations to the other elements, and the 

 ordinary mode of articulation with the body of the second verte- 

 bra, this shows no ' odontoid j^rocess ' in fishes. 



The number of vertebrre varies greatly in the different osseous 

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

 largest: the apodal fishes (Eels, Gymnotes) have the 35 

 most and smallest, in proportion to their size. It is not 

 easy to determine the precise number, on account of the 

 coalescence of some of the vertebras, or at least of their 

 central elements, in particular parts of the column. In- 

 stances of anchylosis of some of the anterior vertebra?, 

 analogous to that noticed in tire cartilaginous Sturgeons, 

 Chlma;ra3, Ehinobatcs, and some Sharks, occur also 

 amongst the osseous fishes, as in many Siiuroid and Cy- 

 prinoid species, in Loricaria and Dactglopterus. Fig. 35 

 represents the four singTilarly elongated anchylosed ante- 

 rior vertebraB in the Tobacco-pipe fish {Fistidaria tahac- 

 caria). A coalescence of several vertebrae is more con- 

 stant at the opposite end of the column in osseous fishes, 

 in order to form the base of the caudal fin, when this is 

 symmetrical in form, as in fig. 33, and in most existing 

 species of Teleostomi. But this modification is arrested 

 at difterent stages in the piscine class. In Cydostomi 

 the gristly parts of the vertebrre continue distinct, with 

 gradual reduction in size to the taper end of the long tail : AnoLjinsta 

 in Protopteri the bony representatives of the caudal ver- tcbnE,' Piue- 

 tebra3 behave in the same way: the notochord jDersists in (Fistuiaria) 

 both orders. In Muraidda, where it is changed into cen- 

 trums, these also gradually diminish in size, and remain distinct 

 to the tail-end. The continuous vertical fold of skin bordering 

 the compressed, long, and slender termination of the vertebral 

 column is not specialised as a caudal fin.' In Plagiostomi, Holo- 

 cepliali, Sturionida, and many Ganoidei, the caudal fin, fig. 

 29, c, is formed chiefly by the htemal spines and appendages, 

 developed to support a lower ' lobe ;' the vertebra? continue 

 distinct to the end of the tail, which bending upward, seems to 

 form an uj^per lobe longer than the lower : to this unsymme- 

 trical tail-fin the term ' heterocercal ' is applied. By decreased 



' This primitive embiyonal basis of the piscine tail-fin is not to be eonfonntled, 

 because it is symmetrical as to shape, with the extreme stage of derelopemental modi- 

 fication constituting the true ' honiocercal ' type of most existing fishes. 



