42 ANATOMY OF VERTEBRATES. 



number, with progressive confluence, of the caudal vertebrae, 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 vertebra; 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 plate, fig. 33, n'ti , from 

 which the neural and haemal arches and their spines 

 radiate : from these elements alone can the number of 

 vertebrae 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 vertebrae. In 

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

 row of rudimental vertebra? had been blended together 

 at rio-lit 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. 



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 

 union of the bodies, c, and haemal spines, As, of the 

 first two of the caudal series, as in fig. 36 ; 1 in which 

 the broad and deep haemal spines are concave forwards, 

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

 In the Halibut (Hippoglossus) the parapophyses of the 

 corresponding vertebrae 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. 



The number of trunk-vertebrae is a useful specific 



character in Ichthyology ; and in counting them the 



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



Sun-fish ( Orthacjoriscus) I find but 8 abdominal and 8 



Sa Tm"nt' £ a caudal vertebrae by distinct bodies. In a Globe-fish 



(Rhombs) ^ Tetrodon) there are 7 abdominal and 10 caudal vertebrae : 



1 Osteol. Collection, Mus. Coll. Chir. No. 188 ; xliv. i, p. 50. 



