110 ESSAY ON CLASSIFICATION. Part I. 



but the Crustacea exhibit, again, the most striking coincidence. Without entering 

 into details, it appears from the classification of Milne-Edwards that Decapods, Sto- 

 mapods, Amphipods, and Isopods constitute the higher orders, while Branchiopods, 

 Entomostraca, Trilobites, and the parasitic types, constitute, mth Limulus, the lower 

 orders of this class.^ In the classification of Dana,^ his first type embraces Deca- 

 pods and Stomapods, the second Amphipods and Isopods, the third Entomostraca, 

 including Branchiopods, the fourth Cirripedia, and the fifth Rotatoria. Both acknowl- 

 edge in the main the same gradation; though they differ greatly in the combina- 

 tion of the leading groups, and also the exclusion by Milne-Edwards of some types, 

 as the Rotifera, which Burmeister first, then Dana and Leydig, unite justly, as I 

 beheve, with the Crustacea.^ This gradation now presents the most perfect coinci- 

 dence with the order of succession of Crustacea in past geological ages, even down 

 to their subdivisions into minor groups. Trilobites and Entomostraca are the only 

 representatives of the class in palaeozoic rocks; in the middle geological ages appear 

 a variety of Shrimb, among which the Macrouran Decapods are prominent, and later 

 only the Brachyoura, wdiich 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 geological formations, though much valuable information 

 has already been obtained respecting the entomological faunjB 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 Radiata. Among these 

 we find their respective classes appearing simultaneously in the oldest periods of 

 the history of our earth. Not so with the Vertebrata, for though Fishes may be 

 as old as any of the lower classes. Reptiles, 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 con- 

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

 as they embody characters, which, in later periods, appear 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 knowTi partake of 

 the characters, which, at a later time, are exclusively found in Reptiles, and no 

 longer belong to the Fishes of the present day. It may be said, that the earliest 

 Fishes are rather the oldest representatives of the type of Vertebrata than of the 



1 Milne-Edwards, Hist. Nat, des Crustaces, " Leydig, (Fr.,) Raderthiere, etc., Zeitsch. f. 



Paris, 1834-40, 3 vols. 8vo. wiss. Zool. 1854, vol. 6, p. 1. 



^ Dana, (J. D.,) Crustacea, q. a., p. 32. ^ Heer, q. a. ; Brodie, q. a., p. 98. 



