544 HISTORICAL PALEONTOLOGY. 



though not altogether rare, are very perceptibly reduced in 

 numbers, the more important forms being Marsupites, Penta- 

 crinus, Bourgueticrinus, and Comatula. Sea-urchins, on the 

 other hand, are so numerous as to constitute one of the most 

 marked features in the Cretaceous fauna. The leading genera 

 are Micraster, Ananchytes, Galerites, Hemipneustes, Diadema, 

 Discoidea, Saletiia, Cidaris, Catopygus, Pygaster, Pygaulus, 

 Holaster, &c. 



The Arthropods are represented by various Crustaceans be- 

 longing mainly to the Macrurous and Brachyurous Decapods, 

 and to the Cirripedes. The little Ostracodes are also abun- 

 dant in many parts of the series, especially in the fresh-water 

 strata of the Wealden. 



Coming to the Mollusca, the Polyzoa have a great develop- 

 ment in the Cretaceous deposits, the family of the Escharida 

 here attaining its maximum. Amongst the more characteristic 

 Cretaceous genera may be mentioned Eschara, Escharina, 

 Vincularia, Membranipora, Flustra, Reticulipora, Homer a, 

 Tubulipora, &c. 



Brachiopods are not especially numerous, and belong mainly 

 to Terebratula, Terebratella, Terebratulina, Rhynchonella, and 

 Crania. Bivalves are very abundant, and some of them are 

 very characteristic. Amongst these are numerous species of 

 Ostrea, Exogyra, Lima, Plicatula, Pecten, and Spondylus, with 

 the various species of Inoceramus, and the great family of the 

 Hippuritida. With the exception of a few Jurassic species, 

 the genus Inoceramus is exclusively Cretaceous, being repre- 

 sented in deposits of this period by numerous species, and not 

 being known to have survived it. The Hippuritidce, or Rudis- 

 tes of Lamarck, comprise a great number of very aberrant 

 Bivalves, all of which were attached and lived associated in 

 beds, like Oysters. The two valves of the shell are always 

 unlike in sculpturing, in appearance, and in shape, and the cast 

 of the interior is often very unlike the form of the outer surface 

 of the shell. A great many species of this family are known, 

 chiefly referable to the genera Hippurites, Radiolites, and Ca- 

 prina. The family appears to be exclusively Cretaceous ; and 

 the most characteristic members of the Cretaceous series of the 

 south of Europe consists of certain compact marbles, which 

 are known as " Hippurite Limestone," from the abundance of 

 shells of this family. 



Gasteropods are not particularly numerous in the Cretace- 

 ous Rocks, and belong chiefly to such modern genera as Turri- 

 tella, Natica, Solarium, Sea/aria, Rostellaria. Dentalium, P/iorus, 

 &c. Along with these are species of the persistent genus Pleu- 



