8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



undoubtedly representing one or more postterminal centra (PC2 of 

 fig. iC), running up below the anterior end of uroneural 2; in the 

 other two genera mentioned above, uroneural 2 has fused with 

 uroneural i and the postterminal centra to form a single monolithic 

 structure. In Brycon there are two epurals; in the others only one. 

 In one feature, the lack of fusion between hypural i and the terminal 

 vertebra, Chasmistes seems to represent the primitive condition. 



With regard to variations other than those mentioned, hypural 2 

 seems to show the greatest plasticity. In Brycon moorei (fig. iC) 

 hypural 2 is separate from the hypurals above and below, but in a 

 U. S. National Museum skeleton of Brycon oligolepis, hypural 2 fuses 

 basally with hypural 3 for a short distance and distally with hypural 

 I. In Chasmistes there is a basal fusion between hypurals i and 2, 

 and in Diplomystes between hypurals 2 and 3. 



As to possible relationships of the order, the caudal skeleton of 

 Brycon bears a striking resemblance to that of the round herring, 

 Dussumieria (Gosline, i960, fig. 7). The upright neural arch of 

 the terminal vertebra of Brycon, Chasmistes, and Diplomystes is a 

 notable feature of the Clupeoidae in general. The fusion of hypural 

 3 with the terminal vertebra is found again in Dussumieria as is the 

 Brycon characteristic of a wedgelike strut running below uroneural 2. 



From the basal types of ostariophysine caudal skeletons described 

 above, the various lineages have evolved different peculiarities. Two 

 of the more specialized types of catfish caudal skeletons have been 

 illustrated by Whitehouse (1910, pi. 47, figs. 7, 8). The characins have 

 presumably given rise to the gymnotid eels, many of which have a 

 tail tapering to a fine filament ; in these, at least, the caudal skeleton 

 has been lost entirely. In the cyprinoids, by contrast, the caudal 

 skeletons seem to remain relatively constant: those of the catos- 

 tomids, cyprinids, and even of the round-tailed cobitid Misgurnus 

 anguillicaudatus, differ only in minor detail. 



In view of this cyprinoid constancy of skeletal structure, the varia- 

 tion in caudal ray count comes as something of a surprise. In all the 

 members of the Cyprinidae examined — Hypophthalmichthys, Garra, 

 Gobiobotia, Saurogobio, Hemibarbiis, and numerous American forms 

 (but not Moapa and Eremichthys, which were unavailable) — the 

 branched caudal rays are constantly 17. On the other hand, most of 

 the cyprinoids examined except the Cyprinidae have fewer than 17 

 branched rays (Psilorhynchus was unavailable). In numerous catos- 

 tomids examined (including Myxocyprinus of China) the branched 

 caudal ray count is constantly 16. Among old world relatives of the 



