THE CRETACEOUS PERIOD. 



275 



that they constitute one of the most marked features of the 

 fauna of the period. From the many genera of Sea-urchins 

 which occur in strata of this age, it is difficult to select char- 

 acteristic types; but the genera Galerites (fig. 191), Discoidea 

 (fig. 192), Micraster, Ananchytes, Diadema, Salenia, and Ci- 

 daris, may be mentioned as being all important Cretaceous 

 groups. 



Coming to the Annulose Animals of the Cretaceous period, 

 there is little special to remark. The Crustaceans belong for 

 the most part to the highly-organized groups of the Lobsters 



Fig. 191. Galerites albogalerus, viewed from below, from the side, and from above. 

 White Chalk. 



and the Crabs (the Macrurous and Brachyurous Decapods) ; 

 but there are also numerous little Ostracodes, especially in the 

 fresh-water strata of the Wealden. It should further be noted 



Fig. 192. Discoidea cylindrica; under, side, and upper aspect. 

 Upper Greensand. 



that there occurs here a great development of the singular 

 Crustaceans family of the Barnacles (Lepadida}, whilst the allied 

 family of the equally singular Acorn-shells (Bafanide) is feebly 

 represented as well. 



Passing on to the Mollusca, the class of the Sea-mats and 

 Sea-mosses (Polyzoa} is immensely developed in the Cretaceous 

 period, nearly two hundred species being known to occur in 

 the Chalk. Most of the Cretaceous forms belong to the family 



