viii INTEODUCTION. 



3. Classification. 



The class Pisces, as understood in this work, includes craniate vertebrates with 

 gills, and typically with median and paired fins which are supported by endoskcletal 

 elements. Three sub-classes are recognized^ viz. Cyclostomata, Selachii, and 

 Teleostomi, which are, however, separated from each other by characters at least 

 as trenchant as those which serve for the distinction of the classes of higher 

 vertebrates, and which have therefore, with some reason, been considered as separate 

 elasses, in which case the term Pisces has been restricted to the Teleostomi. The 

 Teleostomi approach the Batrachians in that they have typically a lung or its 

 homologue, the air-bladder, and in the development of membrane bones. The 

 Batrachia are well separated, however, by the absence of dermal fin-rays and of 

 endoskeletal supports for the median fins, by the modification of the paired fins into 

 pentadactyle limbs and of the hyomandibular into the stapes, and by the presence 

 of true internal nares. 



I especially lay stress on the last character, because the impression is prevalent that 

 the Dipneusti have internal nares, and in this respect show affinity to the Batrachians. 

 As a matter of fact, the Dipneusti, like other Teleostomes, have two external nasal 

 openings on each side ; these are situated on the under side of the snout, and when 

 the mouth is closed the posterior and, to a certain extent, the anterior are covered by 

 the lower lip. The posterior nostril is also covered by the lower lip when the mouth 

 is closed in some Eels (e. g. Ophichthys). 



True internal nares are quite different in structure and position to the posterior 

 external nares of the Teleostomi, being paired perforations of the palate internal to the 

 prsemaxillaries and maxillaries. Their ontogenetic development in the Batrachians, 

 the lowest group in which they occur, supports the view that they are a new 

 formation. 



The Cyclostomes and Selachians are represented by only a few species in the fresh 

 waters of Mexico and Central America, the bulk of the fish-fauna belonging to the 

 Teleostomi, or bony fishes. 



These may be arranged in two series — Actinopterygian and Crossopterygian. In the 

 fishes of the Actinopterygian series the branchiostegal rays and supports of the paired 

 fins retain their primitive condition or evolve by a simple process of concentration or 

 reduction, whilst the duct connecting the air-bladder with the digestive tract, when 

 persistent, opens dorsally or dorso-laterally into the latter. The fishes of the 

 Crossopterygian series comprise the orders Crossopterygii and Dipneusti ; these have 



