Classijication of the Order Ostariophysi. 573 



\\. Anterior rnnms of parnpopliTsis of fmirtli veitobra only distinct 

 distally ; sixili vortL'ura with a laminar transverse process 

 united by suture with tb.it oi" the fifth. 



Jlemisorubim, Pseudoplaty- 

 sloma. 



III. rtervfroid absent ; mesoptpryprnid laminar, attached to latenil 

 ethmoid, but connected with nietapttMTgoid only by ligament; 

 j>jiraj)oj)hysis of fourth vertt'bra a lamina decurved anteriorly, not 

 divided into rami, united with the filth basally ; sixth vertebra 

 free ; air-bladder large, free, partly constricted into two lateral 

 eacs ; head broad, depressed. .. . Pseudopimclodu^. 



IV. Pterygoid absent ; mesopterygoid a small lamina attached to 

 lowt-r surface cf lateral ethmoid and to nietapterygoid, which is 

 produced forward external to it; parapopbysis of fourth vertebra 

 united with fifth, curved to form a semi-cylinder open beliind, 

 containing the lateral portion of the reduced and divided air- 

 bladder ; sixth vertebra not free, but with normal parapophyses. 



Luciopimelodus, 



V. Pterygoid absent ; mesopterygoid small, articulated with posterior 



face of lateral ethmoid and with metapterygoid, which extends 

 forward below the lateral ethmoid ; parapophyses of fourth, fifth, 

 and sixth vertebra? laminar, united by suture ; air-bladder normal. 



Sonibim, Platystomatich thys. 



Family 18. Helogenidae. 



The South American genus Helogenes differs externally 

 from the Pimeloditlje in the tins, as the dorsal is spineless, 

 median in position, followed by a small adipose fia and 

 placed above the long anal, which has 42 rays. The air- 

 bladder is free, of moderate size, placed transversely, and 

 incompletely divided into two compartments. The upper 

 limb of the supra-cleithrum seems to be only loosely attached 

 to the skull and the hypocoracoids do not form a symphysis. 



The relations of this genus appear to be about equally 

 close with the Pimelodidae and with Ceiopsis; one of the 

 types has been partly dissected for examination of the air- 

 bladder, but until a skeleton is available the exact position 

 o^ Helogenes must remain uncertain. 



Family 19. Hypophthalmidae. 



The neotropical genus Hijpophthalmus differs from the 

 Pimelodidae ex.ternally in the toothless mouth, ventro-lateral 

 position of the eyes, very wide gill-openings, very long 

 many-rayed anal Hn, and insertion of the pelvic fins in 

 advance of the dorsal. The lower pharyngeals are united 

 to form a Y-shapcd bone with long anterior stem lying in 

 the narrow isthmus and with small toothed posterior forks. 

 The air-bladder is reduced to two small sacs, each enclosed 

 in a bony capsule with a lateral opening beneath the skin. 



