FISHES CONSIDERED AS A CLASS. 253 



Dr. Theodore Gill regards the group of Fishes not as 

 one class, but as including three Classes, namely 



I. PISCES, as Fishes proper such as Rays, Sharks, Gar-pikes, 



Pipe-fishes, Puffers, Perch, Cod, Tautog, Salmon, etc. 

 II. MARSIPOBRANCHIATES, as Lampreys, Hags, etc. 

 III. LEPTOCARDIANS, as the Amphioxus, etc. 



. classification quite extensively adopted recognizes 

 eleven orders of Fishes. It is essentially that of Miiller 

 as modified by Owen and others ; and this classification 

 we prefer to adopt for our present purposes. In connec- 

 tion with this we also indicate the four groups quite 

 generally recognized Selachians, Ganoids, Teliosts, and 



17. Chondropterygii, with free gills, as Sturgeons. 

 8. " with fixed gills, as Sharks and 



Skates. 

 9. Cyclostomi, as Lampreys, etc. 



According to the earlier writings of Agassiz, Fishes may he divided into 

 four Orders, the scales being taken as a basis of classification, thus : 



1. PLACOIDS or fishes with plates or scales armed with a point, as Sharks 

 and Rays ; 2. GANOIDS or fishes with enameled scales, as the Gar-pikes, 

 etc. ; 3. CYCLOIDS or fishes with rounded scales, as Salmon, etc. ; and 

 4 CTENOIDS or fishes with toothed scales, as Perch, etc. 



A later classification of Agassiz regards the group of Fishes as including 

 four Classes 



I. SELACHIANS, as Sharks, Rays, etc. 

 II. GANOIDS, as Gar-pikes, etc. 



III. FISHES PROPER, Perch, Salmon, etc. 



IV. MTZONTES, as Lampreys, Hags, etc. 



Muller recognizes six Sub-classes of Fishes, viz.: 



I. TELEOSTEI, as all ordinary fishes. 

 II. DIPNOI, as the Lepidosiren. 



III. GANOIDEI, as the Gar-pikes, etc. 



IV. ELASMOBRANCHII, as the Rays, Sharks, etc. 

 V. MARSIPOBRANCHII, as Lampreys, Hags, etc. 



VI. LEPTOCARDII (PHARYNGOBRANCHII), as the Lancelet or Amphioxus. 



Huxley, Rolleston, and some others, regard these six groups as Orders, 

 and place the Dipnoi as the first. Cope recognizes five sub-classes (sec his 

 papers). 



