XxX INTRODUCTION. 
pterygii, which were abundant in Cretaceous seas; no pre-tertiary Ostariophysi 
have been found, but Siluride occur in the earliest Eocene deposits. With 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 
Gymnotide and Cyprinide are evidently modified Characinide, whilst the Loricariidz 
and Aspredinide have been derived from the Siluridea. The Characinide and 
Siluride, 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 CHaracinip& 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 
Distrrpution oF CHARACINID®. 
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 
° e e ? 
but the family is not represented in Madagascar. None of the genera are common. to 
