Class ijicat ton of the Order (Jsiariophys'i. 559 



23-4'l) ; ril)s on transverse processes; eighth or ninth the 

 first free vertebra ; j)arii[)upliysi's of fifth vestigial or absent; 

 ])arai)()[)liyses of fourth iiiKJivided, proxiiually 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 Hshcs. 



The principal genera may be arranged thus : — 



I. A series of bony plates on each side ; mouth rather small ; pelvic 



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

 caecum Doras, O.vi/dora-', Hemiduras. 



II. Body naked ; pelvic fins G- to IG-rayed ; air-bladder large, free, 



normal or with a posterior caecum. 



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



fixed, transverse Cetitroinochlus, Tracheli/opte- 



rus, Trachelyopterichthys, Pseudaitchenipteruf), Aucheni- 

 pierus (Euanemun), Traohyconjstes, Epaptarus, Tetra- 

 nematichthys. 



B. Month very wide, extending far beyond eye ; praemaxillaries 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 

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

 "spring ap])aratus" attached to the free wall of the air-bladder 

 at the larger openings ; mouth wide, tlie movable praemaxillaries 

 extending to its angle Ageniosus. 



Family 4. Plotosidse. 



Body naked, elongate ; gill-menibraiies 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 of vomer and to the large metapterygoid, 

 Avhich is connected with tlie palatine external and the 

 orbitosphenoid internal to its union with the mesopterygoid; 

 head of hyomandibular very broad, articulating with pterotic, 

 spheuotic, 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 ; 



