478 WILLIAM K. GREGORY 



presence of six barbels, (5) the inferior position of the mouth, 

 (6) the fact that the air bladder is in immediate contact with 

 the skin ; and these independently acquired characters seem to 

 indicate the possession of "a potential of similar evolution" by 

 ancestors of each of the groups. 



The order does not present as wide a range of variation as 

 do the Nematognathi, possibly because of its more recent origin. 

 There are four families: (1) The Catostomidse or Suckers' (e.g. 

 Ictiobus) are the more primitive, in that the maxillary forms 

 part of the margin of the upper jaw while the pharyngeal teeth 

 are very numerous. On the other hand, in (2) the Cyprinidae or 

 true Carps, the maxillary does not form part of the margin of 

 the upper jaw, simply assisting in the protrusion of the mouth, 

 while the pharyngeal teeth are reduced in number. About 200 

 genera and nearly 1000 species are known (Jordan). The 

 North American genera while very closely related, are separated 

 by characters which although reasonably constant are often of 

 slight structural importance (Jordan). An interesting speciali- 

 zation is the highly colored breeding dress of the males. (3) The 

 Cobitidas or Loaches (described above). (4) The Homalopteridae, 

 mountain forms with depressed head and horizontally expanded 

 paired fins "which sometimes form a sucking disk. " All members 

 of the order inhabit freshwater. Fossil forms date from the 

 Upper Tertiary and are closely allied to or identical with living 

 genera. The Eventognathi are probably "modern" (Middle 

 Tertiary) offshoots of the ostariophysan stem. The union of the 

 Eventognathi (Carps) with the Heterognathi (Characins) into 

 the order Plectospondyli while justly expressing the ultimate 

 kinship of these two groups arbitrarily separates them from the 

 Nematognathi (Cat fishes). 



Superorder Uncertain. 



Order Apodes 1 LinncBus. 



The Eels. 



(Plate XXIX.) 



Under this name Linnaeus grouped many wholly unrelated forms 



1 a, without, ttovs, foot, from the absence of pelvic fins. 



