DRIVER— THE LUCIOPIMELODIN^. 449 



from each other for more than their distal halves (Plate II, B, 9, 10, 

 5). The first one articulates distally with the clavicle and process 

 of the post-temporal, the second and third are free at the ends. The 

 first and second (9 and 10) belong to the fourth vertebra,^ the third 

 process belonging to the fifth vertebra. In Pseudoplatystoma fas- 

 ciatum there are four expanded transverse processes, all of them 

 separated from each other toward their distal ends. In small speci- 

 mens of this species the interlocking sutures of the centra of the 

 coalesced vertebrae are quite distinct. In this species it is quite evi- 

 dent that the first and second processes belong to the second distin- 

 guishable vertebra, in reality to the fourth. The fourth process 

 belongs to a supernumerary modified vertebra, the sixth, one usually 

 normal in other species. Its processes bear ribs. In Pinielodus 

 clarias the lateral processes are united to form a solid plate without 

 even marginal notches to indicate the boundaries between successive 

 processes. 



In the genera Pimclodina, Pseudopimolodiis, Rhamdia, Pimelodiis, 

 etc., of the Pimelodinee the air-bladder*' is well developed and lies 

 free in the abdominal cavity. It is simple and more or less oblong 

 in shape and extends posteriorly to the eighth or tenth vertebra. 



In Sornhim (Fig. i. A) and Hemisoriibim the air-bladder is 

 drawn out more than in any of the other genera, and extends back 

 past the tenth vertebra. It is larger proportionally than in any of the 

 other genera. In Pseudoplatystoma and BracJiyplatystoma it is very 

 much as in Sortibim and Hemisoruhim but slightly shorter, extend- 

 ing only past the ninth vertebra. 



In Rhamdia (Fig. i, B), Pimelodella and Pimelodus (Fig. 1, C) 

 the air-bladder is very dift'erent from that found in Soruhim, Hemi- 

 soriibim, Pseudoplatystoma and BracJiyplatystoma. In these genera 

 it is wider than long. The width averaging from one to three mm. 



5 Bridges and Haddon, PJiil. Trans., Vol. B, 1893. 



6 The air-bladder of one specimen of Ageneiosus caucanus has been 

 examined to determine its relation to the Pimelodinge, and to the genera 

 Megalonema and Luciopimelodus. This speciinen was 280 mm. long and' its 

 air-bladder was 23 mm. long and 23 mm. wide. It tapers at the posterior 

 end, causing it to be heart-shaped. It lies free in the abdominal cavity and. 

 is not enclosed in a bony capsule. It thus resembles the condition found in 

 the Pimelodinse. 



