PRINCIPAL SYSTEMS OF ZOOLOGY 207 



kind upon which classes are founded. I am also satisfied that the 

 differences which exist between the Selachians (the Skates, Sharks, 

 and Chimasras) are of the same kind as those which distinguish the 

 Amphibians from the Reptiles proper, and justify therefore their 

 separation as a class from the Fishes proper. I consider also the Cy- 

 clostomes as a distinct class for similar reasons; but I am still doubt- 

 ful whether the Ganoids should be separated also from the ordinary 

 Fishes. This, however, cannot be decided until their embryological 

 development has been thoroughly investigated, though I have already 

 collected data which favor this view of the case. Should this expecta- 

 tion be realized, the branch of Vertebrata would contain the follow- 

 ing classes: — 



1st Class: Myzontes; with two orders, Myxinoids and Cyclostomes. 



2d Class: Fishes proper; with two orders, Ctenoids and Cycloids. ^*^ 



3d Class: Ganoids; with three orders, Coelacanths, Acipenseroids, 

 and Sauroids; and doubtful, the Siluroids, Plectognaths, and Lopho- 

 branches.^^ 



4th Class: Selachians; with three orders. Chimeras, Galeodes, and 

 Batides. 



5th Class: Amphibians; with three orders, Casciliae, Ichthyodi, and 

 Anura. 



6th Class: Reptiles; with four orders, Serpentes, Saurii, Rhizo- 

 dontes, and Testudinata. 



7th Class: Birds; with four orders, Natatores, Grallas, Rasores, and 

 Insessores (including Scansores and Accipitres). 



8th Class: Mammalia; with three orders, Marsupialia, Herbivora, 

 and Carnivora. 



I shall avail myself of an early opportunity to investigate more 

 fully how far these groups of Vertebrata exhibit such characters as 

 distinguish classes; and I submit my present impressions upon this 



" I am satisfied that this subdivision of the Fishes proper requires modifications; but 

 I fear it would lead me too far, were I to discuss here the reasons for the changes I 

 propose to introduce into it. 



" I have observed a very curious and peculiar mode of locomotion in all the Lopho- 

 branches, Scleroderms, and Gymnodontes which I have seen alive. They do not pro- 

 gress by the lateral motions of the vertebral columns, as other fishes do; but chiefly 

 by an undulatory movement of their vertical fins, resembling very much the mode 

 of action of the vibratile membranes. In this they resemble the young Lepidosteus; 

 and I consider this fact as a new argument in favor of their association with the true 

 Ganoids. 



