10 THE ENTOMOSTRACA OF 



numerous, and we have not found more than four or five additional forms in the Lias 

 and in the Lower and Upper Oohtes. In the Tertiary Formation, however, and in the 

 recent state the species are far more varied and abundant. 



No L Cythere Hilseana, Hcemer. Tab. I, fig. L a — ff. 



CyTHEBlNA HiLSEANA, Reenter. 1840. Verstein. Norddeutsch. Kreid. p. 104, n. 1, pi. xvi, fig. 17 

 — — Reuss. 1845. Verstein. Bohm, Kreid. p. 16, pi. v, fig. 39 a, b. 



INCH. 



Length, ^ 



Height, 



40 



gi^ 1 Greensand, Blackdown. Eocene Sand, Colwell Bay, Isle of Wight. 



■Jj Gault, Folkstone and Leacon Hill. — Lower Fresh-water Formation, Isle of Wight. 



Thickness, -^-^ -^-^ Chalk-marl, Dover. — Clay, Barton, Hants. 



Detritus, Charing. Miocene, Coralline Crag, Walton. 



Hils-clay, North Germany (Roemer). Chalk-marl and Chalk, Bohemia (Reuss). Chalk, Balsherg, Sweden. 



Carapace sub-triangular. Valves convex, finely punctate, and hairy.^ The internal 

 surface of the valves is also covered with hairs, especially near the dorsal border. 

 Dorsal border elliptical in the left (larger) valve, and rather angular in the right 

 (smaller) valve ; ventral border almost straight. Anterior extremity obliquely semi- 

 circular ; posterior subacute, pointing obliquely downwards. Both extremities are 

 provided with spines, especially on their inferior moieties. 



Dorsal aspect acute oval ; anterior ovate. 



Just within the anterior hinge each valve exhibits a small circular pit, peculiar to 

 this species, marked externally by a corresponding protuberance. In C. Hilseana the 

 hinges are less developed than in the generality of Cytheres, especially as regards the 

 hinge-teeth, which are merely "the knurled extremities of the hinge-bar ; and, contrary 

 to what obtains in other species, the hinges arc equally developed anteriorly and 

 posteriorly. 



This species is abundant in the Gault, and is of frequent occurrence in the Chalk- 

 marl and the Detritus, it is found also in the Greensand ; it is not present, however, in 

 the English Chalk, although occurring in the Chalk of Bohemia and of Balsberg. It 

 is described by M. Roemer as found in the Hils-clay' of Hilse, North Germany. 



An apparently identical species is found in the Lower Fresh-water (so-called) For- 



^ In this and the following instances each column of fractions represents the dimensions of an individual. 



2 The delicate external characters of this and other species are well preserved in the Gault. The 

 specimens from the other Cretaceous strata have their markings and spines more or less worn. 



3 The Hils-clay (Hils-thon), according to M. Rcemer, is the lowest member of the Neocomian series, 

 lying upon the Weald-clay, and representing the Speeton-clay, which latter, in M. Roemer's opinion, belongs 

 to the Neocomian group. (See Translation of Appendix to Roemer's North Germany Chalk Formation, in 

 Taylor's 'Scientific Memoirs,' vol. iv, art. 5.) 



