6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



as the Serranidae, may retain its independent identity ; consequently 

 it is in this line, which will be called Type I, that a simple urostylar 

 structure most frequently occurs. 



A rather different course of evolution seems to have occurred in 

 the herringlike fishes (Clupeidae, Dussumieriidae, Engraulidae), and 

 the caudal structure of these fishes will be called Type II. Here, one 

 or more uroneurals fuse with the terminal vertebra so that the up- 

 turned strut, sometimes called urostyle in these fishes, is really a 

 composite structure made up largely of the uroneurals. (This was 

 noted long ago by Regan, 1910). Additionally, there is a difference 

 from Type I in the way the postterminal centra become fused to the 

 terminal vertebra. As a first step, the base of hypural 3 fuses with 

 at least a portion of postterminal centrum i. As a result, what ap- 

 pears to be the base of hypural 3 interdigitates between the terminal 

 vertebra and the remaining (second) postterminal centrum. Also with 

 the fusion between the base of hypural 3 and the anteriormost post- 

 terminal centrum, hypural 2 loses all basal articulation (as in fig. iC). 

 Later in the evolution of this lineage, postterminal centrum 2 also 

 becomes fused with the uroneural-postterminal centrum complex (as 

 in fig. iC). At a final stage the uroneurals, terminal vertebra, and 

 postterminal centra may fuse into a single structure without indication 

 of origin (as in figs. iD and 2A). 



Though the basic purpose of this paper is to trace morphological 

 similarities in the caudal skeleton as possible indicators of phylo- 

 genetic evolution, a secondary objective is to check certain aspects of 

 the relationship between skeletal structure and fin form. As was 

 noted in the earlier paper, there seems to be an indirect association 

 between tail shape, number of caudal rays, and structure of the caudal 

 skeleton. Since the ostariophysine fishes provide excellent material 

 for investigating certain aspects of this relationship, they will be 

 dealt with first. 



Order Cyprini formes. — Among the ostariophysine fishes the caudal 

 skeletons of the basal members — Brycon (Characidae, fig. iC), 

 Chasmistes (Catostomidae, fig. iD), and Diplomystes (a catfish, fig. 

 2A) — show a surprising amount of general similarity. In none of the 

 three is there a separate postterminal centrum. In all, there are the 

 usual three lower hypurals ; the terminal vertebra has a well- 

 developed, upright neural arch; and tne anterior uroneural and hy- 

 pural 3 are fused with the terminal vertebra. 



In most respects the caudal skeleton of Brycon is the most primitive 

 of the three. Thus, in Brycon there are three uroneurals and a wedge. 



