PREFACE 



The Crustacea of the strata below the Chalk, which form the subject of the present 

 portion of this work, present several remarkable peculiarities in their forms and affinities. 

 One of the most interesting of these is the existence of analogous or, so to speak, 

 representative species in these beds and in the London Clay, on the Crustacea of 

 which I have already treated. In some cases this representation is shown in their 

 specific distinction, with the most perfect generic identity, as in the case of Ho'ploparia, 

 of which we have already seen two very distinct species in the later formation, and we 

 have now described no fewer than six species in the earlier deposits. In no instance do 

 any of these locally separated individuals belong respectively to the same species ; in every 

 one the specific distinctness is unambiguous, but the generic relation to each other is no 

 less so. Another case, of nearly similar import, occurs in the anomurous family Dromiadae ; 

 the Homolopsis of the Greensand being represented in the London Clay, by two species of 

 Bromilites, a very nearly allied form. 



These examples are extremely interesting when associated with the fact that, as far as 

 our present information extends, there is no form in the whole immense bed of the Chalk 

 proper, which at all approximates those which are respectively found in the beds which it 

 separates by such an incalculable epoch and such an enormous space. 



Such facts as these are extremely suggestive, as bearing upon important questions 



