Length, ^ 



A 



Height, ^ 



^V 



Thickness, ^\ 



T^O 



THE CRETACEOUS FORMATION. 11 



mation and in the Eocene Colwell Bay Sands, Isle of Wight, and also in the Coralline 

 Crag. Allied forms occur in the Lias and in the Tertiary beds, both British and 

 foreign. 



No. 2. Cythere punctatula, Roemer. Tab. I, fig. 2 a — n. 



Cytheeina punctatula, Roemer. 1840. Verstein. Kreid. p. 104, n. 2, pi. xvi, fig. 18. 



— coNCENTEiCA, Beuss. 1845. Verstein. Bohm. Kreid. p. 104, n. 17, pi. xxiv, 

 fig. 22, a, b, c. 

 CrxHEEE scuLPTA, Cortiuel. 1846, M6m. Soc. Geol. de Frtuice, 2 ser. torn, i, 2 part. p. 201, 



n. 4, pi. viii, figs. 20-23. 

 Cytheeina concenteica, Williamson. 1847. Trans. Manchest. Phil. Soc. vol. viii. Memoir 



on some, &c., p. 82, pi. iv, fig. 77. 

 Ctpeidina R(EMEEiana, Bosquet. 1847. Entom. foss. Maestricht, p. 12, n. 4, pi. ii, fig. 2 a-f. 



INCH. 



Greensand, Warminster. 



Gault, Folkstone and Leacon Hill. 



Chalk-marl, Dover. 



Detritus, Charing. 



Chalk, South-east of England. 



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



Inf. Greensand, France (Cornuel). Chalk, Maestricht (Bosquet). 



Carapace irregular ovate. Valves strongly convex, especially on the lower or 

 ventral portion ; in young specimens, covered with reticulations, the meshes arranged 

 somewhat concentrically, the ridges or raised part of the network armed with fine 

 spines. In adult shells these ridges lose their spines and become thickened, encroaching 

 on the meshes, until the latter appears as minute and more or less concentric puncta- 

 tions. In very old individuals there remain only the ridges or coarse wrinkles, strongly 

 marked and regular on the ventral part of the valves, but fainter on the dorsal part, 

 and on the central surface broken up into irregular corrugations. Dorsal margin of the 

 left (larger) vahe elliptical ; of the right valve less arched and sinuous ; ventral margin 

 nearly straight, compressed inwards, overhung by the convexity of the valve. Anterior 

 extremity rounded, having a slight, flat, bearded lip ; posterior extremity somewhat 

 narrower than the anterior, shelving to a flat subacute lip. 



Dorsal aspect subacute oval ; anterior sub-cordate. 



The considerable variation of shape and relative size to which the individuals 

 of this species are subject, and the very different eonditions in which the valves occur 

 have been the cause of its receiving three or four distinct appellations. In the Green- 

 sand, the Chalk-marl, the Detritus, and the Chalk the valves are generally much 

 smoothed down or worn, leaving nothing but the coarse wrinkles or the punctations 

 (according to the age of the individual) apparent. In the Gault, however, we find 

 individuals of all ages, and in consequence of the high state of preservation in which 



