THE CRETACEOUS FORMATION. 29 



YOUNG. ADULT. 

 INCH. INCH. 



Length, 

 Height, 

 Thickness, 



5 

 _1 



J77 Greensand, Warminster. 



1 



Gault, Folkstone and Leacon Hill. 



3 4 



-1- Chalk-marl, Dover. 



Detritus, Charing. 



Red Chalk, Flamborough. 



Chalk, South-East England. 



Inferior Greensand, France {Cornuet). Eocene Tertiary, Bracklesham. 



Lower Chalk, North Germany (Rcemer). 

 Chalk, Bohemia (Reuss), 



— Maestricht (Bosquet). 



— Weinbohla, Saxony. 



— Royan, South France. 



Carapace oblong ovate, generally smooth and shining ; spines apparent in some 

 young shells. Individuals vary in shape according to age ; the young shell is ovate, 

 higher on the anterior than the posterior half. The centre of the dorsal margin 

 becomes more and more acutely protruded the older the individual grows, and the 

 ventral margin becomes more or less incurved, giving to some specimens a kidney- 

 shaped form. Occasionally adult specimens of a narrow or elongate oval shape occur 

 (fig. 24 /i), the dorsal margin being but slightly arched. The valves convex ; the 

 convexity slight in young individuals, and increasing with age ; somewhat rounded at 

 each extremity. Dorsal margin of right (large) valve elliptical ; ventral nearly straight, 

 variable. The left valve narrower than the rigid, less arched on the dorsal border, 

 slanting off suddenly at the upper half of the posterior extremity, incurved at the 

 middle of the ventral border, bean-shaped, and readily distinguishable from its 

 fellow-valve.^ 



Dorsal aspect narrow obovate ; anterior oval. 



In this species the groove and flange of the contact-margins are strongly developed; 

 so also is the internal tubercle lying between the centre of the valve and the dorsal 

 margin. 



C. ovata is one of the most abundant species of Entomostraca in the Cretaceous 

 system ; it is most plentiful in the Chalk and Chalk-marl, and we have found it in all 



^ With respect to this and the following species, we differ from M. Bosquet as to which is the dextral 

 and which the sinistral valve, and, consequently, as to which is the anterior and which the posterior extremiijy 

 of the shell. M. Bosquet appears to have been led to regard the obtuse extremity as the anterior, contrary 

 to what obtains generally among the Ostracoda, by the relative size of the valves being the reverse of that 

 of the valves of Cythere in general. This exceptional condition is constant in Cytherella, but is unaccom- 

 panied by any other deviation from the typical characters of Cythere sufficient to warrant us in supposing 

 that the analogy between Cytherella and the other groups is broken in any other respect than in the 

 relative size of the valves. 



