THE CRETACEOUS FORMATION. 19 



a crenulated marginal ridge, bearded. Posterior extremity somewhat angular, with a 

 slightj crenulated, marginal ridge, partially bearded. Superior and inferior margins 

 nearly straight; each bearing a crenulated ridge, which in old specimens becomes 

 spiny, especially on the ventral border, which is raised and keeled. Central space 

 occupied by a prominent tubercle and a straight, crenulated, or interrupted ridge ex- 

 tending backwards. In the young shell the tubercle and ridge are continuous, forming 

 a club-shaped mass. Hinges moderately developed. 



Dorsal aspect somewhat oblong, with its anterior and posterior extremities strongly 

 produced. Ventral aspect somewhat obovate, produced at each extremity. Anterior 

 aspect triangular, five-lobed. 



This species occurs in the Chalk and the Detritus, and is exceedingly abundant in 

 the Gault, in which the largest-sized specimens are found. M. Rcemer describes the 

 young form as C. quadrilatera from the Upper Chalk-marl (the lowest strata of the 

 Soft or Upper Chalk), at Geherden.' M. Reuss, under the name of C. ornatissima, 

 describes two somewhat allied forms from the Lower Chalk and the Chalk-marl of 

 Bohemia, and M. Cornuel describes this species as C Harpa from the "argile astreene" 

 of Haute-Marne. It is present in the Wembohla Chalk, and also in the Upper Oolite 

 (white limestone with flints) at Upway, Dorset. 



No. 6. Cythereis ciliata, Eeuss. Tab. IV, fig. 11 a — U. 



Cythemna ciliata, Eeuss. 1845. Verst. Kreid. p. 104, n. 13, pi. xsiv, fig. 17. 



— ECHINULATA, Williamson. 1847. Trans. Mancliest. Phil. Soc, vol. viii. Memoir 

 on some, &c. pi. iv, figs. 75, 76. 



Greensand, Warmington. 



Gault, Folkstone and Leacon Hill. 



Cbalk-marl, Dover. 



Detritus, Charing. 



Chalk, South-East England. 

 Chalk-marl, Bohemia (Reuss). 

 Carapace oblong. Valves obliquely convex, strongly raised at the posterior third of 

 the ventral border, depressed towards the anterior and dorsal borders ; surrounded by 

 a marginal ridge, which is crenulated in the young shell, and becomes more and more 

 rough with age, until in old specimens the valves are edged with coarse spines, 

 strongest on the ventral border ; this border increases in thickness posteriorly until it 

 terminates abruptly opposite to the posterior hinge ; it is strongly keeled, and is armed 

 with six or seven stout spines, the terminal spine double, larger and more acute than 

 the others, pointing obliquely downwards and outwards. 



The surface of the valves of young individuals is coarsely punctated ; in older 



1 Set* Taylor's Scient. Mem., vol. iv, art. 5. 



Length, 



YOUNG. 

 INCH. 



1MM.A 

 INCH. 



TURE. 

 INCH. 



is 



ADULT 

 INCH. 



-V ' 



Height, 



1 



^V 



iV 



4^ 



Thickness 





tV 





1 



