Classification of the Order Ostariophysi. 559 



23-41) ; ribs on transverse processes ; eighth or ninth the 

 first free vertebra ; parapophyses of fifth vestigial or absent ; 

 parapophyses of fourth undivided, proximally thin, laminar, 

 distally forming expanded subvertical oval plates, free from 

 the supra-cleithra, applied to the lateral walls of the anterior 

 part of the air-bladder. 



South American fresh-water fishes. 



The principal genera may be arranged thus : — 



I. A series of bony plates on each side ; mouth i-ather small ; pelvic 



fins 7-rayed ; air-bladder large, free, normal or with a posterior 

 caecum Doras, Oxydoras, Hemidoras. 



II. Body naked ; pelvic fins 6- to 16-rayed ; air-bladder large, free, 



normal or with a posterior csecum. 



A. Mouth rather wide, not extending beyond eye ; prsemaxillaries 



fixed, transverse Ce7itromochlus, Trachelyopte- 



rus, Trachelyopterichthys, Pseudauchenipterus, Aucheni- 

 pterus {Euanemus), Trachycorystes, Epaptertis, Tetra- 

 nematichthys. 



B. Mouth very wide, extending far beyond eye ; prsemaxillaries ex- 



tending to angle of mouth .... Asterophysus. 



III. Body naked ; pelvic fins 7-rayed ; air-bladder very small, partly 

 enclosed in two small spherical or pyriform capsules separated by 

 a septum and attached to the aortic ridges, with small posterior 

 aud larger supero-lateral apertures ; the reduced plates of the 

 ** spring apparatus " attached to the free wall of the air-bladder 

 at tlie larger openings ; mouth wide, the movable prsemaxillaries 

 extending to its angle Ageniosus. 



Family 4. Plotosidaa. ' 



Body naked, elongate ; gill-membranes free or narrowly 

 attached to the isthmus. Dorsal fin well developed, with a 

 spine, placed anteriorly ; no adipose fin ; anal very long, 

 confluent with the caudal, which may extend forward along 

 the back, simulating a second dorsal fin ; pelvics 10- to 16- 

 rayed. Nostrils remote, the posterior with a barbel ; on 

 each side one or two maxillary and two mandibulary barbels. 

 Teeth in jaws conical or obtuse ; a patch of blunt molars on 

 the vomer. Palatine rod-like ; mesopterygoid rather small, 

 attached to head o£ vomer and to the large metapterygoid, 

 which is connected with the palatine external and the 

 orbitosphenoid internal to its union with the mesopterygoid; 

 head of hyomandibular very broad, articulating with pterotic, 

 sphenotic, and alisphenoid. Skull depressed ; sphenotics 

 projecting laterally but not produced forwards, the frontals 

 with free margins. Post-temporal present ; lower fork of 

 supra-cleithrum well developed, attached to basioccipital j 



