Natural History of the United States. 209 



fifth Botatoria. Both acknowledge in the main the same grada- 

 tion ; though they differ greatly in the combination of the lead- 

 ing groups, and also the exclusion by Milne-Edwards of some 

 types, as the Botifera, which Burmeister first, then Dana and 

 Leydig, unite justly, as I believe, with the Crustacea. This gra- 

 dation now presents the most perfect coincidence with the order 

 of succession of Crustacea in past geological ages, even down to 

 their subdivisions into minor groups. Trilobites and Entomo- 

 straca are the only representatives of the class in palaeozoic rocks ; 

 in the middle geological ages appear a variety of Shrimps, among 

 which the Macrouran Decapods are prominent, and later only the 

 Brachyoura, which are the most numerous in our days. 



The fragmentary knowledge we possess of the fossil Insects, 

 does not justify us, yet, in expecting to ascertain with any degree 

 of precision, the character of their succession through all geo- 

 logical formations, though much valuable information has already 

 been obtained respecting the entomological faunae of several 

 geological periods. 



The order of succession of Vertebrata in past ages, exhibits 

 features in many respects differing greatly from the Articulata, 

 Mollusks, and Badiata. Among these we find their respective 

 classes appearing simultaneously in the oldest periods of the his- 

 tory of our earth. Not so with the Vertebrata, for though Fishes 

 may be as old as any of the lower classes, Beptiles, Birds, and 

 Mammalia are introduced successively in the order of their relative 

 rank in their type. Again, the earliest representatives of these 

 classes do not always seem to be the lowest; on the contrary, 

 they are to a certain extent, and in a certain sense, the highest, 

 in as far as they embody characters, which, in later periods, ap- 

 pear separately in higher classes, (See Sect. 26,) to the exclusion 

 of what henceforth constitutes the special character of the lower 

 class. For instance, the oldest Fishes known, partake iof the 

 characters, which, at a later time, are exclusively found in Bep- 

 tiles, and no longer belong to the Fishes of the present day. It 

 may be said, that the earliest Fishes are rather the oldest repre- 

 sentatives of the type of Vertebrata than of the class of Fishes, 

 and that this class assumes only its proper characters after the 

 introduction of the class of Beptiles upon earth. Similar rela- 

 tions may be traced betweens the Beptiles and the classes of 

 Birds and Mammalia, which they precede. I need only allude 

 here to the resemblance of the Pterodactyli and the Birds, and to 



