Vili INTRODUCTION. 
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 Jast 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 
premaxillaries 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 
