EIGENMANN: THE FRESHWATER FISHES OF BRITISH GUIANA 119 



dd. 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 basioccipital; gills lobate; body armed with dermal plates. . . Lophibranchii. 

 ii. Snout not tubiform; parietals present; pterotic not reaching basioccipital; 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 asymmetrical Heterosomata. 



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



Order NEMATOGNATHI. 



Key to the Families of Nematognathi. 

 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 vertebra? 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 villiform. Nares remote. Dorsal short, over the ventrals. Pectoral 



witli a strong spine Aspredinidae, II. 



bb. Opercle well-developed. Gill-openings usually wide. Caudal vertebras not compressed, the 

 neural spines spine-like. Maxillary a vestige. 

 c . Air-bladder well-developed, simple or with transverse constrictions, 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 very long, its origin 



far behind the vertical from the dorsal fin SUuridae, 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, which 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 basioccipital. An adipose fin. Anal of 34 or more rays. 



Hypophthalmidae, 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 Tridens. Air- 

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



of the coalesced vertebra? only. Anal short Pygidiida?, VI. 



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

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

 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. 



