vi PREFACE. 



which have recently occupied anew the attention of naturalists, and they deserve to be 

 well considered and compared with analogous examples, wliich doubtless present themselves 

 in other formations and in other groups of organized beings. 



The investigation of any fossil species is often met by a difficulty which demands some 

 consideration, as showing how necessary it is that the inexperienced palaeontologist should 

 not be prejudiced by the striking discrepancy which is often found in the appearance 

 of different specimens of the same species, from different localities. As the animals are 

 ordinarily foimd in a very imperfect and often in a fragmentary condition, this caution 

 becomes the more important ; and in no group of animals is the difliculty greater than 

 in the Crustacea, the specific, and even tlie generic characters of which are often so subtle 

 or so minute, as to require the closest observation of a practised eye to appreciate them. 

 The mineral character of the bod in Mhicli the specimens are found is so strongly 

 impressed upon them, that a limb from one locality, an abdomen from another, and a 

 carapace from a tiiird, can scarcely be recognised by one unaccustomed to the work as 

 even possibly belonging to an identical species. This is strikingly shown in Hoploparia 

 scabra, which is found under extremely various aspects in the Gault, the Greensand of the 

 neighbourhood of Cambridge, and that of Wiltshire. 



In the present part I liavc the opportunity of describing a third species of the 

 oxyrhynchous form, of which group, as before observed, not one was known in a fossil 

 state when Professor Milne Edwards published his great work on the Crustacea. The 

 first ever ascertained was the Mithracia libitiioides of the former part of this work ; Mr. 

 Charles Gould afterwards described a second, MiUiracites vectensis, found in the lower 

 Greensand of the Isle of Wight; and tlic third, Trachynotm Wea^w-s, a very remarkable 

 form from the upper Greensand of Wiltsliirc, is now for the first time described.^ 



In the various collections of the Crustacea of the Gault and Greensand, to which I 

 have enjoyed the freest access, there is one circumstance wliich is extremely tantalising. 

 This is the occurrence of numerous specimens of the limbs of these animals, often in a 

 very perfect state, Init found in situations so totally separated from any other important 

 part that it is impossible to appropriate them to any hithcn-to known species, or to describe 

 tlicni with any certainty as belonging to any determinate genus. This is particularh' the 

 fact in Mr. Carter's fine collection of Cambridge Greensand Crustacea and in that of Mr. 



1 Page 2, pi. i, %. 2. 



