x INTRODUCTION. 
The fishes of the next sub-order, Ostariophysi, differ from the Malacopterygii in 
the modification of the superior and lateral elements of the anterior vertebra to form 
a chain of ossicles connecting the air-bladder with the auditory organ. 
This group is almost entirely composed of fresh-water fishes, and five of the six 
families recognized by Boulenger are represented in Mexico and Central America. 
The Ostariophysi agree with the Malacopterygii in having a mesocoracoid bone, but 
in the other groups to be considered this element has been lost. 
Of four groups of more or less anguilliform fishes, Lyomeri, Heteromi, Symbranchii, 
and Apodes, which appear to be independent offshoots of the Malacopterygii, the last 
two are represented in the rivers of Mexico and Central America. 
In the Symbranchii the body is eel-shaped, the dorsal and anal fins are continuous 
with the caudal, the ventral fins are absent, the gill-openings are confluent to form 
a single ventral slit, and the well-developed premaxillaries exclude the maxillaries 
from the border of the mouth. 
The Eels of the sub-order Apodes differ in having the small gill-openings usually 
separate and in having the mouth bordered above by the maxillaries, which are 
separated in the median line by the ethmo-vomer, to which the premaxillaries 
(if present) are suturally united. 
The sub-ordinal groups already mentioned appear to be natural and well-defined, 
but the next sub-order, Haplomi, which differs from the Malacopterygii only in the 
absence of a mesocoracoid, is less satisfactory, and includes four or five groups which 
may not be related. 
The Cyprinodontide are abundant in the rivers of Mexico and Central America. 
Recent researches have shown that some members of this family are physoclistic *, and 
that in others the lower pharyngeal bones are suturally united. On this account 
I would place the physoclistic Scombresocide, which have the lower pharyngeals 
completely united, near the Cyprinodontide in the sub-order Haplomi. In my 
opinion they are much more closely related to the Cyprinodontide than to the 
Atherinide, with which they have recently been associated. 
Most of the physoclistic Teleosts with abdominal ventral fins are placed by 
Boulenger in two sub-orders, Catosteomi and Percesoces, the former being distinguished 
from the latter by the supposed enlarged coracoid. In the ‘Cambridge Natural 
History’ Boulenger writes:—‘“'The whole question of the arrangement of the 
* Philippi, Sitzungsb. Gesellsch. naturf. Freund. 1906, p. 232, 
