EIGENMANN: the freshwater fishes of BRITISH GUIANA 119 



(Id. Duct of air-bladder closed. 



h. Ventral fins sub-abdominal; lower pharyngeals separate. 



i. Snout tubiform; mouth terminal, toothless; parietals absent; pterotic extending down- 

 ward to basioecipital; gills lobate; body armed with dermal plates. . . Lophibranchii. 

 a. Snout not tubiform; parietals present; pterotic not reaching basioecipital; body not 



armed; ventral fins of a spine and five rays Percesoces. 



hh. Ventral fins thoracic. 



j. Gill-openings large; a spinous dorsal, or the dorsal very long. 



k. Cranium symmetrical Percomorphae. 



kk. Cranium asj-mmetrical Heterosomata. 



jj. Gill-opening small; maxillary and premaxillary united; no ribs Plectognathi. 



Order NEMATOGNATHI. 



Key to the Families of Nem.\tognathi. 

 a. Body naked or with a single series of plates along the middle of the sides. Mouth terminal or sub- 

 terminal. 

 b. Opercle minute, a mere vestige; neural spines of the coalesced vertebrae forming a ridge from the 

 occipital to the dorsal. Caudal vertebrae greatly compressed, their neural spines expanded. 

 Gill-openings reduced to a slit in front of the pectorals. Air-bladder well-developed. No 

 adipose fin. Teeth villiforni. Nares remote. Dorsal short, over the ventrals. Pectoral 



with a .strong spine Aspredinidae, II. 



66. Opercle well-developed. Gill-openings usually wide. Caudal vertebra; not compressed, the 

 neural spines spine-Uke. Maxillary a vestige. 

 c. Air-bladder well-developed, simple or with transverse con.strictions, lying free in the abdominal 

 cavity. Body naked or with a single series of plates along the middle of the sides; 

 an adipose fin. 



d. Dorsal over abdominal portion of the vertebral column; anal not verj^ long, its origin 



far behind the vertical from the dorsal fin SUuridas, III. 



dd. Dorsal over caudal portion of the vertebral column; anal very long, nearly co-extensive 

 with the tail, its origin far in advance of the vertical from the dorsal; air-bladder 



transversely elongate, reniform, not covered by bone Helogeneidae, IV. 



cc. Air-bladder double, a minute one on either side of the coalesced vertebra;. 



e. A small adipose fin. Dorsal over the anal, wliich is very long, origin of anal near the 



origin of the second third of the length. Air-bladders enclosed by the lateral 

 processes of the coalesced vertebra;, the scapula and the process connecting the 

 scapula with the basioecipital. An adipose fin. Anal of 34 or more rays. 



Hypophthalmidse, V. 

 ee. No adipose fin; anal short, posterior; dorsal usually in advance of the anal, sometimes far 

 back and over the short anal or behind the origin of the anal in Tridcns. Air- 

 bladder double, a minute one on each side, enclosed by the lateral processes 



of the coalesced vertebra; only. Anal short Pygidiidae, VI. 



aa. Sides with two series of plates; caudal vertebra normal, the neural and haemal spines spike-like, 

 separate from each other. Air-bladders minute, one on either side of the coalesced vertebrae 

 and surrounded by a bony capsule, in contact with the skin at a long narrow slit in the temporal 

 plate and at the last of a series of slits below the longer one. Mouth terminal; teeth villiform. 



Callichthyidae, VII. 



