INTRODUCTION. 



The Tertiary Entomostraca of Britain have hitherto received but little 

 notice. In my 'Monograph of the Cretaceous Entomostraca/ 1849, published by the 

 Palaeontographical Society, I referred to the occurrence of some of the Cretaceous 

 species in our Tertiary deposits. 1 Dr. Reuss, about the same time, figured and 

 described two forms which he had received from the Barton Clay of Hampshire ; and I 

 have since described a few species from Woolwich and Col well. 2 Several of the Post- 

 tertiary fresh-water Entomostraca, however, have been noticed by me, in 1 850, in the 

 ' Annals of Natural History.' To these latter species I have now to add two others ; 

 and of the Tertiary Entomostraca I am enabled, from my own collection and with the 

 aid of my friends, to enumerate 50 species ; altogether amounting to 58 species. Of 

 these, 30 (including three described by me in the ' Memoirs of the Geological Survey ' 

 whilst preparing this Monograph) are new. 



The Post-tertiary species are included in this Monograph for the sake of con- 

 venience, and to render the subject as perfect as circumstances will permit. The 

 recent forms which have become fossilized in the Post-tertiary deposits, either of fresh- 

 water, estuarine, or marine origin, not only complete the geological series up to the 

 present day, but are of considerable value in assisting us in the identification of some 

 carapace-remains, otherwise obscure in their relations. The list of these later species 

 of Ostracoda may be also regarded as a not uninteresting addition to the valuable 

 resume of the Molluscan fauna of the Post-tertiary and Pleistocene eras, with which 

 Mr. S. V. Wood has enriched his elaborate and valuable ' Monograph on the 

 Mollusca of the Crag.' 



1 In a few instances the determination of the species or of the deposits has since appeared to me to be 

 unsatisfactory: the localities, therefore, given in the present Monograph are to be considered as corrected 

 in such cases. 



2 With respect to the Colwell species which -were inadvertently quoted as from Woolwich, see further 

 on, p. 26. 



