X INTRODUCTION. 



The fishes of the next sub-order, Ostariophysi, differ from the Mahrcopterj'gii in 

 the modification of the superior and lateral elements of the anterior vertebrce 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 praemaxillaries 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 

 sepai'ated in the median line by the ethmo-vomer, to which the prtemaxillaries 

 (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 Cyprinodontidse 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 Scombresocidaj, which have the lower pharyngeals 

 completely united, near the Cyprinodontidai in the sub-order Haplomi. In my 

 opinion they are much more closely related to the Cyprinodontidse than to the 

 Atherinidse, with which they have recently been associated. 



Most of the physoclistic Teleosts with abdominal ventral fias 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 



* Thilippi, SltzuDgsb. Gcscllsch. naturl'. In-cund. lOuG, p. L'32. 



