THE ENTOMOSTRACA 



THE CRETACEOUS FORMATION. 



Of the numerous microscopic organisms afforded by the disintegration of the 

 OoUtic, Cretaceous, and Tertiary strata, whether calcareous, argillaceous, or sandy, 

 the Polythalamia and the Entomostraca are by far the most abundant. The latter 

 attract obsei'vation by their larger relative size, but have not been hitherto very 

 extensively described. 



Our present observations are confined to the Entomostraca of the Cretaceous 

 Formation, comprising the Chalk, Chalk-marl, Gault, and Greensand. The strata that 

 have afforded the species herein described are the greensand beds of Blackdown 

 (siliceous), of Warminster (calcareous), and of Farringdon (arenaceous) ; the Speeton 

 Clay ; the Gault of Folkstone and of Leacon Hill, near Charing ; the Grey Chalk and 

 Chalk-marl of Dover ; the Cretaceous Detritus of Charing ; the Red Chalk of Flam- 

 borough ; and the common White Chalk of the south-east of England. In the above- 

 mentioned strata these little fossils lie about irregularly in all directions, and are not 

 found in layers such as are often formed by the Entomostraca of the Wealden and 

 Tertiary beds ; many are broken or eroded, and some bear evidence of having been 

 crushed whilst in a recent state. The animal matter of the carapace is well preserved 

 in the Gault specimens, and more or less so in others ; individuals bearing a trace of 

 transparency and of original colouring occur, but are extremely rare, in the Chalk ; 

 whilst in the Gault a large proportion remains transparent.^ The specimens generally 

 occur as separate valves, but perfect carapaces are not unfrequently found, the latter 

 condition being dependent upon the strength of the hinge attachment, which varies 

 considerably in different species. 



The Gault and the Soft Chalk are very rich in Entomostraca ; but the most abundant 

 supply of these and other Cretaceous animalculites is obtained from the " Chalk- 



1 MM. Cornuel and Bosquet refer to the fact of the colouring matter being retained by some of the 

 Entomostraca in the French Neocomian beds and the Maastricht limestone. 



B 



