574 Mr. C. T. Regan on the 



formed by the parapophyses of the fourth and fifth vertebrae 

 and by the upper limb of the supra-cleithrum. The vertebrae 

 number 66 (14 + 52), the first five ankylosed ; the ribs are 

 inserted on parapophyses. 



Family 20. Trichomycteridae. (Fig. 2, B.) 

 Body naked, moderately elongate. Gill-openings wide or 

 restricted. Dorsal fin short, without spine ; no adipose fin ; 

 anal short or moderately long ; pelvics 5- or 6-rayed. 

 Nostrils remote, the anterior sometimes with a barbel ; 

 teeth in jaws villiform or incisor-like; palate sometimes 

 toothed. Palatine broad ; pterygoid absent ; mesopterygoid, 

 when present, A^erj' small, attached to inner edge of meta- 

 pterygoid, wdiich joins palatine. Skull depressed, contracted 

 between and expanded behind tb.e orbits. Post-temporal 

 absent ; upper limb of supra-cleithrum suturally united to 

 supraoecipital and pterotic ; mesocoracoid present ; hypo- 

 coracoids narrowed forward below, pointed anteriorly, not 

 interlocking. Vertebrae 42 to 46 (19-21 + 23-25) ; ribs on 

 parapophyses; fifth vertebra ankyio^ed with the complex, 

 which is short, rigidly attached or ankylosed to the skull, 

 and bears a pair of transverse subcylindrical capsules with 

 lateral openings ; air-bladder divided into two lateral sacs 

 enclosed in the bony capsules. 



Freshwater fishes of South Ameiica. 



The principal genera may be arranged thus : — 



I. Air-bladder capsule formed by the parapopbysis of the fourth 



vertebra, iu front free from the skull aud normally attached to 

 the supra-cleithrum, behind free from the normal parapopbysis of 

 the tifth vertebra ; dorsal fin anterior ; opercles unarmed ; one 

 pair of maxillary barbels. {Ceto2}sin(S.) 



Cetopsis. 



II. Air-bladder capsule united to the slaill, its anterior wall including' 



the lower limb of supra-cleithrum and a part of the exoccipital 

 and epiotic bones ; parapopbysis of fifth vertebra not developed 

 asasepaiate process; dorsal tin median or posterior. {Tric/io- 

 im/derince.) 



A. Opercles unarmed ; one pair of maxillary barbels. 



Nematoyenys. 



B. Operculum and interoperculum armed with spines. 

 One pair of maxillary barbels SteciophUns, Vanclellia. 



Two pairs of maxillary barbels Trkhomycrerus, EremopJiibis, 



Tridenfi, Pariodon, Miuroylanis. 



Family 21. Bunocephalidae. ' 

 Gill-openings reduced to a small foramen in front of the 

 pectoral fin. Dorsal fin anterior; no adipose fin; caudal 



