MANUAL OF NATURAL HISTORY. 569 



on the skin; the SuccJwsaurus with biconcave, and 

 the Streptospondylus with convexo-concave ver- 

 tebrae. Besides these, there were several genera 

 of Marsh Tortoises, as Tretosternon, Euryster- 

 non, Trionyx, Platemys and Emys. The fishes of 

 this group comprise the genera Lepidosteus, Lepi- 

 dotus and Hybodus. The remains of the little 

 entomostracous genus Cypris are very abundant in 

 the Wealden of Kent, and of the Isle of Wight, 

 their bivalve carapaces occurring in extensive layers ; 

 the crustaceous genus, Archceoniscus, is also from 

 the Wealden of France and England, and numerous 

 remains of Coleopterous, Neuropterous, Orthop- 

 terous, Dipterous, and Hemipterous insects have 

 likewise been detected. 



Among mollusca the fresh-water genera Cyclas, 

 Unio, Paludina, and Anodonta, have been met with. 



Y. — Cretaceous Group. 



The Chalk group comprises a series of marine 

 deposits, composed of chalk, limestone, sandstone, 

 marls, and clays. The lower portion, or gait, con- 

 sists of beds of bluish or black clay, with green 

 sand, containing hardened concretions ; the upper 

 portion, or chalk proper, consists of soft, white 

 chalk, with layers of flint, and hard chalk without 

 flint. The chalk formation composed the bed of an 

 ancient ocean, and contains the organic remains of 

 many marine plants and animals. It extends over 

 portions of the British Islands, various parts of 

 France, Germany, Denmark, Sweden, Russia, and 



