INTKODUCTION. 



In examining the zoological characters of the Crustacea which existed during the 

 Eocene period, it is impossible not to be struck by the fact, that notwithstanding the 

 obvious relation in most of them to the members of recent groups, amounting often to an 

 almost typical representation of a family, there is at the same time, probably without 

 exception, such a discrepancy as forbids their association under the same generic formula. 

 In some few cases, indeed, it has been difhcidt, in consequence of that general destruction 

 of the minute but very important organs about the oral, antennary, and ophthalmic regions, 

 which too often prohibits any very correct appreciation of the relations of the species, 

 to assign to the extinct form its true place amongst its recent allies. But ordinarily there 

 have been, amongst the numerous specimens which I have been so fortunate as to 

 have placed at my disposal, some which have sutficed to indicate their affinities with 

 great probability, if not with absolute precision ; and these have led me to adopt the 

 conclusion above stated, which is somewhat at variance with the recognised relation 

 between the Eocene and more recent forms of most other classes of animals, of which 

 i-epresentatives are found even in that early member of the tertiary series. 



In making the necessary comparisons, and in endeavouring to assign to the different 

 characters and structiu^es their absolute or relative importance, I have found it requisite to 

 investigate with some care the homologies of the different regions and subordinate portions 

 of the carapace, as it happens too often that we are driven to that part, almost exclusively, 

 us the basis of our diagnosis. I will, therefore, make a few remarks on this important 

 element in our generaUzation, before I enter upon the detailed application of these data to 

 the determination and description of the objects themselves. 



'I'he first distinct attempt at an anatomical division of the carapace into "regions," 

 on the basis of their presumed relation to the viscera which they respectively cover, 

 was made by Professor Desmarest, in his portion of the ' Histoiie Naturelle des Crustaces 



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