INTRODUCTION. 



Vll 



abiuidance in the first or Sheppey zone of Mr. Prestwich. This is the case even 

 with those species which are common to most of the districts of the London Clay, 

 pai-ticularly the two most common species of Xanthopsis. The genera Mitliracia, 

 Xantfiilites, Placjioloplms, Portunites, QSdisoma, Campi/lostoma, and Goniochele, and the 

 Macrurous genus Scyllaridia are, as far as I have observed, confined to this zone. There 

 is one species, Ci/clocorysier pidchellus, which has hitherto occm-red only in the third zone, 

 a single specimen having been found at HoUoway. Hoploparia yaviniaroides is far more 

 numerous at Sheppey than in either of the other localities, but it also occurs in the 

 Highgate and Hollovvay districts. Hoploparia Bellii, on the contrary, is far more numerous 

 in the North London than in the Sheppey zone, and appears to be equally common 

 to both the second and third zones of Mr. Prestwich. Finally, Archmocarahus Boxcerhanhii 

 is principally, but not exclusively, a Sheppey species ; a few specimens from HoUoway 

 existing in Mr. Wetherell's collection. I subjoin a list of the species described in the 

 present work, with the localities in which they occur. 



The other districts in which the London Clay exists have not been hitherto examined 

 with any degree of care and diligence ; and it is probable that there ai'e yet several 

 species which have not been discovered, and which will reward the researches of some 



future investigator. 



