Natural History of the United States. 341 
3d Class : Ganoids; with three orde 
and Sauroids; and doubtful, the Siluroids, Plectognaths, and Lopho- 
es 
4th Chas: Selachians; with three orders, Chimere, Galeodes, 
and Batides. 
‘thts Class: Amphibians; with three orders, Cacilie, Iethyodi, and 
ra, 
6th Class: Reptiles; with four orders, Serpentes, Saurii, Rhizo- 
dontes, and Testudinata. 
ith Class: Birds; with four orders, , Natatores, Grallz, Rasores, 
sessores, (including Scansores and Accipitres. : 
8th Class: Mammali a; with three orders, Marsupialia, Herbivora, 
and Carnivora.” 
_ Here then, at the very first step, there will probably be a divi- 
“1 among naturalists as to the signification of the principles 
laid down. Some will assume, and correctly as we believe, that 
the adult form of fishes and reptiles is the true expression of 
the potentialities of the type, and should alone be regarded in 
whole Tange of structure through development. It is obvious 
too that there is room for wide diveaaity in the use of the other 
_ Subdivisions, down to genera and even s 
pecies. 
my Wgain, if the subdivisions of fishes above mentioned are called 
asses, some may ask, what name shall be applied to this entire 
sToup, the rank of which would be between Branch and Class 
,. Whatever may be the final conclusion on these points, the 
discussion which has been pursued by Professor Agassiz has 
eady borne science to a higher level than it had before attained, 
and he a force and direction to thought which will insure 
Tap progress towards perfection. The work proceeds next with 
M8 special topic, the North American Testudinata ; and the sub- 
J€ct is carried out with that thoroughness of research and beauty 
of illustration reaching even to the structure of the embryo and 
c 
*Pen to discussion. Moreover, the volumes by Agassiz which 
are to follow in the series, will make still broader the foundation 
fer the true philosophy of nature. We shall look eagerly for 
the last words on this subject from his searching mind. 
€ review of the Embryological part of the volumes forms a 
Agassiz’s assistant, the results of whose coca 3k investiga- 
