xii INTEODUOTIOX. 



to truly thoracic ventral fins [vcith the pelvic bones directly attached to the clavicles) 

 in the series OjiMoceplialm, Anahas, Osjyhromenus and Tetrcujoiiurus, Centroloplms, 

 Stromateus respectively, and in order to attain precise diagnoses it becomes necessary 

 to regard each of these two groups as of primary, i. e. sub-ordinal, rank, and to neglect 

 the character of the attachment of the pelvic bones in defining them. 



The first and second of these sub-orders have the names Percesoces and Labyrinthici 

 respectively. For the third I propose the name Zenopharyngii, whilst the fourth, 

 comprising fishes with the skeleton in great part cartilaginous, may be termed 

 Malacichthyes. Of these groups only the Percesoces is represented in the fresh waters 

 of Mexico and Central America. 



The sub-order Acanthopterygii, as understood by Boulenger, is defined by a single 

 character, viz., tlie direct attachment of the pelvic bones to the pectoral arch. When 

 the systematic portion of the present work was commenced, more than two years ago, 

 the sub-order was accepted by me ; but I now think that the fact has to be 

 recognized that the pelvic bones have become directly attached to the clavicles in 

 several groups and that a large group defined by this character alone is unnatural. 



It has been mentioned above that each of two natural groups, Zenopharyngii and 

 Labyrinthici, includes forms with the pelvic bones remote from the clavicles and 

 others in which they have acquired a direct attachment to the clavicles. In a third 

 group, the Berycoids, the two conditions are also found, Polijniixia exemplifying the 

 first and TracliicJitliys the second. It seems pretty certain that the Cyttidte and 

 tlie Percoid fishes have evolved independently from the Berycoids, and we cannot 

 lay much stress on the fact that the pelvic bones are directly attached to the clavicles 

 in the two first-named groups as indicating any special relationship between them. 



Enough has been said to show that the Acanthopterygii should be split up into 

 several sub-orders ; two of these are represented in the fresh waters of Mexico and 

 Central America, viz. Percomorphi and lieterosomata, the latter including the sin<>ie 

 family Pleuronectida;. 



4. Geographical Distribution. 

 In discussing the distribution of fresh-water fishes we need not take notice of 

 marine fishes {Centropotmis, Pomadasis, Mugil, &c.) which ascend rivers for con- 

 siderable distances, either in search of food or for breeding-parposes, nor need we 

 consider fishes which descend the rivers to breed in the sea (e. g. AnqnlUa) 

 Fresh-water species of marine genera (e. g. Cldrostoma humloldtidnum) and fresh- 

 water genera of marine families (e. g. Xenatlicrina) have little importance. True 



