iv INTRODUCTION. 



ship being maintained for adjoining figures. It had been intended to include the 

 series in three plates, but fresh species were discovered, and these, together with 

 forms in which it was desirable to illustrate particular features on a larger scale, 

 are shown on the fourth plate on the scale of 25 and 30 diameters. 



As regards the classification, we have followed as closely as possible that 

 adopted by Brady and Norman in their excellent Monograph of the Marine and 

 Freshwater Ostracoda of the North Atlantic and North-west Europe (' Trans. 

 Royal Dublin Society,' ser. 2, vol. iv, 1889). Of course with respect to the fossil 

 forms, the character of the carapace-valves furnishes the only ground for comparison 

 with the recent ; but this is in most instances now known to be so intimately cor- 

 related with the other structural features of the organism, that it may reasonably 

 be considered as affording a safe clue to their systematic relationships. 



We are enabled to give the geological horizon and locality from which the 

 fresh material studied by us has been obtained with geater precision than was 

 possible in the former Monograph, and we append hereto some notes of the strata 

 in which the number and variety of species have been most marked. 



§ I. Upper Chalk. — Horstead, Norfolk. — The Chalk in this locality belongs to the 

 zone of Belemnitella mucronata; and, if we except the beds at Trimingham, it is on 

 the highest horizon of the formation in this country. In addition to the horizontal 

 layers of flints, which occur here the same as in the Upper Chalk of other places, 

 there are numerous larger flint masses, subcylindrical to subspherical in form, in 

 some instances with open tubular cavities, in others with central cavities com- 

 pletely inclosed by a flinty crust. These masses, known as " Paramoudras "^ or 

 " Pot-stones," range up to three or four feet in length and from one to three feet 

 in diameter. The interior cavities of these stones are in some cases filled with a 

 hard porous mass of silica, whilst in others there is a quantity of fine powdery 

 material, resembling in appearance the Chalk itself, but it is incoherent, and, un- 

 like the Chalk, it is for the most part siliceous in composition. This powder or 

 "flint-meal," as it has been termed, is usually made up to a great extent of Fora- 

 minifera, Entomostraca, and other minute organisms, of which the Chalk is largely 

 formed, and there is also in it a great number of the spicules of siliceous Sponges, 

 likewise originally in the Chalk. The composition of this flint-meal may be re- 

 garded as representing to a great extent the structure of the Chalk whilst in the 

 condition of a deep-sea ooze, before pressure and other subsequent changes of 

 fossilization had consolidated the material and crushed together its component 

 organisms. 



Completely sealed up within one of these pot-stones in the Chalk at Horstead, 

 one of us obtained, some years since, a quantity of flint-meal, which proved to be 



1 S. Woodward, ' Outlines of the Geology of Norfolk,' 1833, p. 2G ; H. B. Woodward, ' Geology of 

 England and Wales,' 2nd ed., 1887, p. 399. 



