4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 42 



members of the less aberrant groups are compared, as will be done 

 here, a more stable picture emerges. 



In a previous paper (Gosline, i960) the caudal skeletons of iso- 

 spondylous fishes were discussed. There it was demonstrated that the 

 "primitive" members, e.g., Elops, Salmo, Hiodon, and Esox, have a 

 large number of features of caudal structure in common, and that this 

 basic type has evolved in various ways within the order. The present 

 paper deals with the caudal skeleton in certain of the orders usually 

 placed between the isospondylous fishes (Clupei formes) and the 

 percomorph fishes (Perci formes). All the caudal structures to be dis- 

 cussed parallel one of three of the types developed within the iso- 

 spondylous fishes. By way of background these three types will be 

 redescribed. 



Here, as in the earlier paper, the "terminal vertebra" {TV of figs.) 

 is defined as the one bearing a laterally flanged lower hypural (HYi 

 of figs.) ; this, in turn, is the lowest of the (typically) three hypurals 

 articulating with the lower lobe of the caudal fin. In the percomorphs, 

 as well as in most of the fishes discussed in this paper, the terminal 

 vertebra forms the posterior end of the vertebral column, but in the 

 lower teleosts there may be one or two separately ossified centra be- 

 hind it. If so, these are called "postterminal centra" {PT of fig. i). 

 When two are present, as in the elopoid Pterothrisstis (fig. lA), 

 hypurals 2 and 3 attach to the anterior (PTi) and one or more hy- 

 purals of the upper caudal lobe to the posterior {PT2). 



Basically it is the varying fate of these two postterminal centra 

 that distinguishes the three types of isospondylous skeletons. Usually 

 these centra become incorporated in the terminal vertebrae in ad- 

 vanced forms, but in one group of isospondylous fishes, the osteo- 

 glossoids, a different fusion has taken place. There, a typical inter- 

 vertebral articulation is retained between postterminal centra i and 

 2, and the latter seems to have fused with a large block presumably 

 formed at least in part from the upper hypurals (fig. iB). Later in 

 the paper this caudal structure will be called Type III. 



The other two types incorporate the two postterminal caudal centra 

 into the terminal vertebra through different sequences of fusion. The 

 usual method is to add the anteriormost first. At this point the three 

 lower hypurals all attach to the terminal vertebra, leaving only a few 

 of the upper hypurals articulating with the remaining postterminal 

 centrum. In a second step, the second (posterior) postterminal cen- 

 trum also fuses with the terminal vertebra (fig. 4A). In this line of 

 development it is notable that the uroneural structure, even as far up 



