30 Infusoria. 



type, however, does not go beyond the period of the coal for- 

 mation, when it is replaced by gigantic Entomostraca, which 

 are in some degree the precursors of the Macruri. Ento- 

 mostraca of small size likewise appear in very ancient for- 

 mations ; they abound in certain coal formations, for exam- 

 ple, and they are found after that in a multitude of deposits. 

 They have not yet, however, been studied in a satisfactory 

 manner. 



The Macruri, with which MM. H. de Meyer and Count 

 de Miinster are particularly occupied, prevail from the tri- 

 assic epoch to the present creation ; while the Brachyura are 

 essentially tertiary. These latter, as well as the Cirripedia, 

 which appear to be their contemporaries everywhere, are still 

 far from being so well known as we would desire. A mono- 

 graph of the Cirripedia, both living and fossil, is in particular 

 a pressing desideratum for zoology as well as for palaeonto- 

 logy. The other orders of Crustacea are not known except 

 in the tertiary formations. The parasitic Crustacea, soft and 

 vermiform, appear to be exclusively confined to the present 

 creation. 



It follows, from this hasty glance, that the types whose 

 affinities have been best studied, such as the Trilobites, Mac- 

 ruri, and Brachyura, succeed each other in the series of for- 

 mations in the order of their organic gradation. It is even 

 very curious to observe the intimate analogy which exists 

 between the forms of these different types and the phases of 

 the embryonic development of the Crustacea, which MM. 

 Rathke and Erdl have afforded us the means of becoming ac- 

 quainted with. 



If I have not hitherto spoken of the Infusoria, it is not be- 

 cause I forget their influence in the history of the formation 

 of our globe. On the contrary, I think that M. Ehrenberg 

 has opened a new era for palseontological researches by his 

 important discoveries in the world of these infinitely minute 

 creatures ; but I likewise think that the novelty of these re^ 

 suits, as surprising as unexpected, do not yet allow us to 

 appreciate them at their just value. 



After having thus passed in review the principal classes of 

 invertebrate animals, whose fossil remains have been most 



