INTRODUCTION. 



When the Monograph of the Cretaceous Bntomostraca was written by one of 

 us in 1849, our knowledge of these organisms, though considerably beyond the 

 rudimentary stage, was very deficient in that critical acquaintance with the struc- 

 tural features which now determine the bounds of genera and species. It is not 

 surprising, therefore, that, with the great amount of research which has been 

 bestowed on the group within the last forty years, it should be found necessary 

 to revise in many respects the determinations then arrived at. The author of the 

 former Monograph can now see that, actuated by a desire not to increase the 

 number of species unnecessarily, he ventured to place some of the English forms 

 under species, described by Continental palseontologists, which have since been 

 proved to be distinct ; and, further, in view of the results of recent researches, 

 that the limits then assigned to genera and species were together of too wide a 

 character to be now maintained. Owing also to the imperfect figures and lax 

 descriptions of species, some of the Cretaceous forms were inaccurately compared 

 with species from different geological formations on the Continent and elsewhere, 

 as well as with some still existing forms. A brief revision of the Monograph of 

 the Cretaceous Entomostraca was attempted in 1870;' but, owing to various ob- 

 stacles, the carrying out of the wish, then expressed, of re-examining the work 

 and reproducing the illustrations on a smaller scale has been delayed until now. 

 In some respects this delay has been advantageous ; for, thanks to recent dis- 

 coveries, not only has the number of species been materially added to, but more 

 complete specimens of forms already known have been obtained, from which better 

 figures have been prepared. 



Of the new plates accompanying this Supplement, the figures on three are 

 drawn mostly on the scale of about 18 diameters, suitable figures in the plates 

 of the Monograph having been reduced by photography and redrawn on this 

 smaller scale. Figures of new species, some on the scales of 20 and of 25 dia- 

 meters, were added, and, owing to the necessities of space, without close relation- 



' " Notes on the Cretaceous Entomostraca " by Prof. T. Rupert Jones, F.G.S., ' Geol. Mag.' vol. vii, 

 1870, pp. 74-77. 



