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FIRST DIVISION, FOURTH CLASS.— PISCES 



PROFESSORS AGASSIZ AND MULLER'S CLASSIFICATION OF FISHES. 



The classification of Fishes proposed by Cuvier has been found to require considerable modification, 

 in consequence of the more complete knowledge since obtained regarding their internal structure ; and 

 also, because it is found inapplicable to the arrangement of the numerous extinct forms which Geo- 

 logical research has brought to light. The attentive study of these has suggested to Professor Agassiz 

 a method of arrangement founded upon their scaly covering, which affords characters well adapted for 

 an easy subdivision of the class, and which enables us to assign a place with little difficulty to the 

 numerous fossils which the examination of even a single stratum often brings into view ; the scales 

 being usually among the best-preserved parts of the entire animal, and bemg often in a state of perfect 

 preservation, when every part of the internal skeleton has disappeared by decay. As this classifica- 

 tion is much in use at the present time, especially amongst those who are engaged in the study of 

 Fossil Fishes, we shall give a brief sketch of it; although, as we shall show, it is far from being 

 perfect. The entire class is divided into the four following orders : — 



I. Ganoideans; from the Greek yavos, splendour. The fishes of this Order have a complete bony armour, 

 usually covered by a coating of enamel, which gives them a peculiarly lustrous appearance. This armour 

 generally consists of scales of small size, and of angular shape, somewhat overlapping each other, and arranged 

 with great regularity, as in the Lepidosteus, or Bony Pike. In other instances, it is composed of plates of large 

 size and irregular shape, with jagged edges that lock together, as in the Sturgeon. The former of these fish has 

 a hony internal skeleton, and the latter a eartUagiiwus one; and a similar difference appears to have existed among 

 the several extinct tribes of the Order. 



II. Placoideans; from the Greek 7rXa£, a hroad plate. This Order contains the fishes whose skin is covered 

 irregularly with plates of hard bony matter, or of enamel; these are sometimes of large size, but are more fre- 

 quently reduced to small points, as where they form the slmgreen on the skin of many Sharks, and the prickly 

 tubercles of the skin of most Rays. In this group, which comprehends the Sharks and Rays and their allies, 

 the skeleton is for the most part cartilaginous. 



III. Ctenoideans; from the Greek KTeis, (Gen. Krevos), a comb. These fishes have scales composed of horny 

 matter, or of bone destitute of enamel, each scale being composed of several layers arranged one beneath an- 

 other. They are distinguished from those of the next order, in which the structure of the scale is nearly the 

 same, by having the posterior edge (that which is directed towards the tail of the fish, and which overlaps the 

 succeeding scale,) beset with projections like the teeth of a comb. The Perch may be taken as the type of this 

 Order. 



IV. Cycloideans ; from the Greek x.ukXi>s, a circle. The scales of these fish have a rounded form, with 

 smooth and simple edges. The Carp, Salmon, and Herring, are familiar examples of this Order, 



If we compare this classification of Agassiz with that of Cuvier, we shall find that the Cycloid fishes of the 

 former are for the most part the Malacopterygii of the latter; and that the Ctenoid fishes of the former are, 

 speaking generally, the Acanthopterygii of the latter. Further, the Placoid fishes of Agassiz correspond with the 

 principal section of the Cartilaginous fishes of Cuvier; the Sturgeons and Chimajra being alone excepted. The 

 existing Ganoid fishes of Agassiz, however, were distributed by Cuvier amongst several different families; and 

 there can be no doubt that, in bringing them together, Agassiz has effected an important improvement in classi- 

 fication, since they present a general correspondence in internal structure, as well as in the nature of their 

 external covering. 



The application of this method of arrangement to the various forms of extinct fishes which Geological 

 research has brought to light, has given some extremely curious results ; of which a sketch will now be given. — 

 In the first place it may be stated as a general fact, that of the Cycloid and Ctenoxd Orders, there are no remains 

 whatever in any formation anterior to the Chalk ; and that, consequently, the whole assemblage of existing fishes 

 included in those two orders, probably about four-fifths of those now living, had apparently no representative 

 whatever in the more ancient seas. Even in the chalk there beem to have been only two or three of the largest 

 of the existing families, such as the Herring and Salmon tribes, the Mackerel tribe, and the Perch tribe, which 

 attained any considerable importance. The others are either but slightly represented at that epoch, and have sub- 

 sequently increased very considerably, such as the Eels and the Pleuronectid<e ; or first came in during the Tertiary 

 period, such as the Carps and the Mullets ; or present themselves for the first time in our own own epoch, which is 

 the case (strange to say) with the large and important Cod tribe. Further, no family belonging to these Orders 



