240 PATAGONIAN EXPEDITIONS: ZOOLOGY. 



In Diplomyste only short maxillary barbels are developed, and, as stated 

 above, the maxillary is dentiferous and forms the mouth border above. It 

 therefore has its barbels less specialized than the remaining families, while 

 its maxillary is more generalized and resembles that of other fishes. The 

 Diplomystidce may therefore be considered the most primitive of the 

 Nematognathi. At present this family is restricted to the fresh waters of 

 central Chili and northern Patagonia. 



The next important factor to be considered is the relative development 

 and specialization of the air-bladder and Weberian apparatus. In the 

 BimocephalidcE and the Siliiridce, with the exception of the Ageiieiosince, 

 the air-bladder is large and lies below the coossified vertebrae. In the 

 remaining South American families the air-bladder has been split into 

 lateral halves, and, with the corresponding bones of the Weberian appa- 

 ratus, has been enclosed in a bony capsule. 



In the BitnocephalidcE there are no indications of any approach to the 

 enclosed air-bladder, and indeed this family shows little resemblance to 

 the remaining families. It was early differentiated from the descendants 

 of the Diplomysfidcc, but not before the maxillary had become quite ves- 

 tigia]. Two subfamilies have become differentiated ; they differ chiefly in 

 the number of vertebrae in the tail and the number of anal rays. This 

 family inhabits *the La Plata basin, the Amazon and the South American 

 rivers to the north of it. 



The cosmopolitan family, Silnridcs, is composed of a large number of 

 subfamilies, seven of which are American, or have representatives in 

 America. They vary-considerably from the most generalized Tachisurinae, 

 which most resemble the Diplomystidce, to the Doradinae and Ageneiosinae, 

 in which last we observe an approach to the conditions obtaining in the 

 families with closed air-bladders. 



As is intimated above, the Tachisurinae are the lowest of the living 

 SiluridcB. The members of this subfamily are mostly marine, and from 

 it have, in all probability, been derived the remaining subfamilies. Some 

 members of the Tachisurinae have only the maxillary barbels developed, 

 while in others the mental barbels have been added. 



From the Tachisurinae have unquestionably been derived the Callo- 

 physinae and Pimelodinas, which differ from each other in dentition. Both 

 these subfamilies are neotropical in their distribution, the Callophysinae 

 being found in the Amazon and northward, the Pimelodinae in the whole 



