164 ESSAY ON CLASSIFICATION. 



satisfy everybody that among Cephalopoda the Tetra- 

 branchiata are inferior to the Dibranchiata ; and it is not 

 too much to say, that one of the first points a collector of 

 fossils may ascertain for himself is the exclusive pre- 

 valence of the representatives of the first of these types in 

 the oldest formations, and the later appearance, about the 

 middle geological ages, of representatives of the other 

 type, which at present is the most widely distributed. 1 



Of Worms, nothing can be said of importance with 

 reference to our inquiry ; but the Crustacea again ex- 

 hibit the most striking coincidence. Without entering 

 into details, it appears, from the classification of Milne- 

 Edwards, that Decapods, Stomapods, Amphipods, and 

 Isopods, constitute the higher orders ; while Bran- 

 chiopods, Entomostraca, Trilobites, and the parasitic 

 types, constitute, with Limulus, the lower orders of this 

 class. 2 In the classification of Dana, 3 his first type 

 embraces Decapods and Stomapods, the second Am- 

 phipods and Isopods, the third Entomostraca including 

 Branchiopods, the fourth Cirripedia, and the fifth Eota- 

 toria. Both authors acknowledge, in the main, the same 

 gradation ; though they differ greatly in the combination 

 of the leading groups, and also in the exclusion, by Milne- 

 Edwards, of some types, as the Rotifera, which Burmeister 

 first and then Dana and Ley dig, justly, as I believe, united 



C. R., Paris, 1841, 4to. MACDONALD, M.,) QUATREFAGES, (An. DE,) et 



(J. D.,) On the anatomy of Nautilus BLANCHARD, (En.) Voyage en Sicile ; 



umbilicatus, compared with that of Paris, 3 vols. 4to. fig., without date. 



Nautilus Pom2)ilius, Trans. Roy. Sc.; 1 Some Ammonites, and especially 



London, 1855, II, p. 277. VAN DER the splendid Crioceras of Bogota, 



HOEVEN, (J.,) Beitrag zur Anatomic described by Valenciennes, exhibit 



von Nautilus Pompilius, L., besonders plainly a combination of characters 



des mannliehen Thieres, Arch, fur found separately in Nautilus and in 



Naturg., 1857, I, p. 77. CUVIER, Argonauta. 



(G.,) Mumoires pour servir a 1'His- 2 MIL^E-EDWARDS, Hist. Nat. des 



toire et a, 1'Anatomie des Mollusques ; Crustaces ; Paris, 1834-40, 3 vols. 8vo. 



Paris, 1817, 4to. fig. EDWARDS, (H. 3 DANA(J.D-), Crustacea, q.a.,p. 45. 



