12 The Evolution of the Catfishes. [ ZOE 
lent guide to the determination of the rank any family is to occupy 
in the system. : 
Other structures valuable in this respect are the different barbels 
so highly developed in some forms. Of the development of the 
barbels of /ctalurus albidus, Ryder says: 
‘‘ The remarkably developed barbels of the embryos of the species 
make their appearance very early, especially the maxillary pair; 
these appear on the second day. * * The barbels on the lower 
jaw do not appear till the fourth day of development is completed. 
* * The last ofvall to be developed is the nasal pair * * [which] 
does not appear until the seventh day.” 
In Diplomystes 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 re- 
sembles most that of other fishes. The Diplomystidz may there- 
fore be considered the most primitive of the Nematognathi. At 
present this family is resricted to the fresh waters of central Chile. 
The next important factor to be considered is the relative devel- 
opment and specialization of the air bladder and Weberian apparatus. 
In the Bunocephalidz and the Siluridz, with the exception of the 
Ageneiosinz, the air bladder is large and lies below the codssified 
vertebre. In the remaining families the air bladder has been split 
into lateral halves, and, with the corresponding bones of the Web- 
erian apparatus, has been enclosed in a bony capsule. 
In the Bunocephalide there are no indications of any approach 
to the enclosed air bladder, and indeed this family shows little re- 
semblance to the remaining families. It has lost the opercular bones 
entirely, and has in other ways attained a high state of specializa- 
tion. It was early differentiated from the descendants ot the Diplo- 
mystidz, but not before the maxillary had become quite vestigiary. 
Two subfamilies haye become differentiated ; they differ chiefly in 
the number of vertebre in the tail and the number of anal rays. 
This family inhabits the Amazon.and the South American rivers to 
the north of it. 5 
The cosmopolitan Siluride is composed of a large number of sub- 
families, seven of which are American, or have representatives in 
America. They vary considerably from the most generalized Tachi- 
surinee which most resemble the Diplomystid to the Dorandinz 
Poe, 
i 
y 
