CLASS IV.-PISCES. 



(The True Fishes.) 



Skeleton bony or cartilaginous. Skull with sutures ; a lower jaw pres- 

 ent ; membrane bones developed in connection with the skull ; gill-open- 

 ings a single slit on each side, sometimes confluent ; gills attached to bony 

 arches, normally four pairs of them, their outer margins free. Median 

 and paired fins usually developed; a lyriform shoulder-girdle. Exo- 

 skeleton various. Ova comparatively small. No "claspers". Brain- 

 differentiated and distinctly developed. Heart developed, divided into 

 an auricle, ventricle, and arterial bulb. 



As here understood, this group includes the great majority of recent 

 fishes, and is equivalent to the Tcleostei, Ganoidei, and Dipnoi of most 

 recent writers, the Actinopteri, Crossopterygia, and Dipnoi of Professor 

 Cope. Omitting all notice of the Dipnoi and the Crossopterygia, all the 

 members of which groups are extralimital and confined to the fresh waters 

 of the tropics, it will be convenient to divide the American representa- 

 tives of the class of Pisces into four series or subclasses Chondrostei, 

 Holostei, Physostomi) and PhysocUsti, the first and second including most 

 of the Ganoidei of Miiller, the second and third the Teleostei. These 

 groups are evidently of unequal value, the Physostomi and the Physo- 

 clisti being very intimately connected, and the relations of the Holostei 

 with the Physostomi are probably more intimate than their relations 

 with the other Ganoids, as has been shown by Professor Cope. Never- 

 theless, these groups exist in nature, and their recognition under one 

 name or another is convenient. 



ANALYSIS OF THE SUBCLASSES OF PISCES. 



a. Arterial bulb muscular, with numerous valves ; optic nerves forming a chiasnia ; 

 ventral fins abdominal ; air-bladder with a duct ; tail heterocercal. 



(Series GANOIOEI.*) 

 6. Ventral fin with an entire series of basilar segments; skeleton cartilaginous. 



CHONDROSTEI. 

 66. Ventral fin with the basilar segments rudimental; skeleton bony.. HOLOSTEI. 



* ydvof, splendor ; from the enamelled scales. 



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