l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



tent to which fusion has occurred differs within and between the 

 various groups of fishes. This secondary simplification of structure 

 has been evolved through a number of steps, and the sequence of these 

 steps seems to have been essentially the same in various groups of 

 fishes. Thus, hypurals 2 and 3 seem to fuse before hypural i becomes 

 added to the group, e.g., in cyprinodonts and apparently in eels ; post- 

 terminal centrum i becomes amalgamated with the terminal vertebra, 

 with postterminal centrum 2 coming in in the more advanced forms ; 

 etc. When these sequences are changed, other features of the caudal 

 skeleton seem to evolve in other than the usual fashion, or vice versa. 

 In any event the different sequences form the bases upon which the 

 three caudal skeleton types designated here have been established. It 

 may be added that the end point in the evolution of all three would 

 be a simple platelike caudal skeleton which would be essentially the 

 same regardless of what sequence had been followed. Such an end 

 point is represented by Fundulus (fig. 3C). 



In a very general way there is an association between fusion of 

 parts in the caudal skeleton and reduction in the number of caudal 

 rays. Perhaps no significance should be read into this other than as 

 independent exemplifications of the trend in the evolution of teleostean 

 fishes toward the reduction of the number of parts throughout the 

 head and body. Certainly in the example of the cyprinoids dealt with 

 above there was no relationship between the number of caudal rays 

 and of hypurals. 



There seems to be equally little direct relationship between the 

 amount of fusion and fin type or mode of life. There are numerous 

 round-tailed fishes with a fused, platelike caudal skeleton {Fundulus) 

 and fork-tailed forms with similar features (Beloni formes) ; similarly 

 there are slow- and weak-moving forms (pipefishes) and fast-swim- 

 ming fishes (tunas) with platelike caudal skeletons. The epitome of 

 consolidation is undoubtedly reached in the small, neotenic Schindleria 



(Gosline, 1959, P- 75)- 



Turning to the three basal types of caudal structure distinguished 

 in the introduction to this section. Type I is by far the commonest. 

 It occurs among all iniomous and in the basal berycoid, zeoid, and 

 perciform fishes. In these the uroneural(s) do not fuse with the 

 vertebral elements and a separate second postterminal centrum is 

 usually visible, though postterminal centrum i has fused with the 

 terminal vertebra. This type is also well represented among iso- 

 spondylous fishes, e.g., salmonoids and stomiatoids. Indeed Type I 

 seems to be a stage in the normal or at least usual route of evolution 

 followed in the modern teleosts. 



Types II and III, by contrast, appear to represent divergent evolu- 



