42 ANATOMY OF VERTEBRATES. 



number, with progressive confluence, of the caudal vertebras, the 

 ' upper lobe ' becomes gradually reduced in length, until the 

 symmetrical shape is attained. But this coexists in the Salmon, 

 Perch, and many extinct Ganoids with an unsymmetrical bend 

 of the coalesced caudal vertebras into the base of the upper lobe. 

 In true ' homocercals ' the terminal bodies of the caudal vertebrae 

 are not separately established in the primitive notochord, but are 

 continuously ossified to form a common, compressed, vertically 

 extended, and often bifurcated bony pjlate, fig. 3.3, rih' , from 

 which the neural and hasmal arches and their spines 

 radiate : from these elements alone can the number of 

 vertebras of such caudal fin be estimated ; normal de- 

 velopement proceeding here in the peripheral elements, 

 as throughout the vertebral column in Lepidosiren, whilst 

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

 the Sun-fish ( Orthagoriscus mala) it would seem as if a 

 row of rudimental vertebras had been blended together 

 mif ^^ right angles to the rest of the column, in order to 

 support the rays of the short, but very deep caudal 

 fin, which terminates the suddenly truncated body of 

 this oddly shaped fish. 

 is{; It is rare to find anchylosis save at the ends of the 



vertebral series in fishes : sometimes, however, in the 

 Pleuronectidce, a kind of sacrum is formed by such bony 

 imion of the bodies, c, and hasmal spines, hs, of the 

 first two of the caudal series, as in fig. 36 ; ' in which 

 the broad and deep liamal spines are concave forwards, 

 and form a sort of pelvic posterior wall of the abdomen. 

 a In the Halibut {Hippor/lossus) the parapophyses of the 



corresponding vertebras with those of the last abdominal 

 are similarly united, though the bodies remain distinct. 

 In Loricaria both the upper and lower arches of a con- 

 siderable part of the caudal region are blended together 

 into an inflexible sacrum ; but, as a general rule, there 

 exists no such impediment to the lateral inflections of 

 the tail in the present class. 



Tlie number of trunk-vertebras is a useful specific 

 character in Iclitliyology ; and in counting them the 

 coalesced caudals are usually reckoned as ' one.' In tlic 

 Sun-fish (Orthiu/orisnis) I find but 8 abdominal and 8 

 caudal vertc1n-;T3 )iy distinct bodies. In a Globe-fish 

 ( Tetrodon) there are 7 abdominal and 10 caudal vertebrx : 



' Cslcol. CuUputiou, Mils Cull. Cliir. No. 188 ; xliv. /. p. 50. 



