198 W. S. MacLeay on the Natural Arrangement of Fishes, 



the most typical of fishes ought therefore to be the most imperfect of 

 them, i. e. the furthest removed from the type of Vertebrata. Such, 

 fishes are evidently the Cyclostomi of Cuvier, such as Myxine, and 

 other genera leading off to Annulosa. Though essentially aberrant, 

 as they relate to vertebrated animals, the Cyclostomous fishes are 

 typical as respects the circle of fishes. Now it is this circle of fishes 

 in which we have the above three data, namely, the two aberrant or- 

 ders and one typical order : consequently I arrange the class as fol- 

 lows, into orders : — 



ABERRANT GROUP. 



CTENOBRANCHII. Gills pectinated. 



1. PLAGIOSTOMI, Cm. Cartilaginous fish with fixt branchia, 



leading to Mammalia. 



2. STURIONES, Cm. Cartilaginous fish with free branchiae. 



3. OSTINOPTERYGII, MacL. Bony fish with free branchia, leading 



to Amphibia. 



NORMAL GROUP. 



ACTENOBRANCHII. Fish breathing by gills not pectinated. 



4. LOPHOBR ANCHII, Cm. Bony fish breathing by tufts arranged 



in pairs along the branchial arches. 



5. CYCLOSTOMI, Cm. Cartilaginous fish breathing by a se- 



ries of cells. 



Now this arrangement differs from that of Swainson, in making 

 the vast majority of fishes an aberrant group ; but it is the structure, 

 not the number of species it contains, that determines the place of a 

 group in nature. The group Ungulata is just as important now, when 

 containing comparatively few genera, as it was in the antediluvian 

 ages, when it contained an immense number of them. Besides, I 

 will venture to say, that the above circular arrangement of fishes 

 expresses their place among Vertebrata better than that of Swainson. 

 I shall differ from him still further as I go on. But in the mean time 

 I must observe, that the above and following new names are merely 

 used in order that you may the better understand my meaning. I 

 have been obliged to invent a technical name for bony fishes with 

 pectinated gills, viz. 



OSTINOPTERYGII, 

 which may thus be divided into tribes : — 



ABERRANT GROUP. 



ACANTHOPTER YGII, Artedi. Spines in first dorsal hard.— Qwgre. Are 

 all these Ctenodians of Agassiz? 

 l.BALISTINA. Plectognathi, Cwr. Maxillary bones soldered to the 



intermaxillaries, and both to the 

 palatine arch. Opercula and gills 

 concealed under the skin. 



2. PERCINA. Bones of the jaws free and complete. 



Operculum distinct. Operculum 

 or preoperculum generally with 

 dentated edges, or with spines. 



3. FISTULARINA. Bones of the jaws free and complete. 



Operculum distinct. Operculum 

 and preoperculum generally with 

 smooth edges. 



