XX 



INTRODUCTION. 



pterygii, ^Yhich were abundant in Cretaceous seas ; no prae-tertiary Ostariophysi 

 have been found, but Siluridse occur in the eailiest Eocene deposits. A¥ith the 

 exception of the Silurid genera Arius, Plotosus, and a few allied to them, which have 

 become estuarine or marine, all the members of the group are fresh-water fishes. The 

 Gymnotidse and Cyprinida^ are evidently modified Characiuid;^, whilst the LoricariidiB 

 and Aspredinidae have been derived from the Silurid;^. The Characinidte and 

 Siluridae, however, are by no means closely related, although evidently derived from 

 the same stock, as is shown by the presence in both of the remarkable chain of 

 " Weberian ossicles" connecting the air-bladder with the auditory organ. The origin 

 and distribution of the Mexican and Central-American families of this group may now 

 be discussed. 



The family Characinid^ is very abundant in the fresh waters of South America, where 

 more than 500 species are known to occur and where the genera are numerous and 



DrsimmjTioN or CnAKAcjNiri^'E. 



very diversified. The most generalized types also occur in this continent, which must 

 be regarded as the original home of the group. In Africa there are about 100 species 

 but the family is not represented in Madagascar. None of the genera are common to 



