482 MESOZOIC TIME REPTILIAN AGE. 



of chalk often contains more than a million of microscopic organ- 

 isms, among which far the most abundant are these Rhizopods 

 (called also Foraminifera and Polythalamia). Some of the species 

 are represented in figs. 778-781. 



The Sponges, also, were of great importance in the history of the 

 Cretaceous rocks. They occur cup- or saucer-shaped, tubular, 

 branched, and of other forms. One is figured in fig. 777. Their 



Spicula of Sponges. 



siliceous spicula are common in the flint, and have contributed, as 

 well as Diatoms, towards the silica of which it was made. 



Among Radiates, the Corals and Echinoids are mostly of modern 

 types, and are far more numerous than in the Cretaceous of 

 North America. 



The same genera of Mollusks abound that are enumerated on 

 p. 472. But the variety of Brachiopods, Gasteropods, and Am- 

 monites is vastly greater than on the American continent. The 

 Ammonites, and the uncoiled forms of the same family mentioned 

 on p. 472, are particularly abundant. One English Ammonite 

 (the A. Lewesiensis), from the Lower Chalk, has a diameter of a 

 yard. 



The genera of Gasteropods are to a greater extent modern 

 genera than in the preceding period, and the proportion of siphon- 

 ated species (having a beak) is nearly as great as in existing seas. 

 The Rudistes (figs. 782-784) are very common in southern Europe 

 and Asia Minor, and about eighty species have been described ; only 

 a single species — Radiolites Mortoni — has been found in England. 



In the sub-kingdom of Vertebrates, there are Fishes of the mo- 

 dern order of Teliosts, and Sharks of the modern tribe of Squalo- 

 donts, as stated with regard to America (p. 473). One of these 

 new fishes of the former group is shown in fig. 791. The Salmon 

 and Perch families are represented among these earliest of Teljosts. 

 Cestraciont teeth are very common. 



The class of Reptiles in the earlier part of the Cretaceous period 

 included the Iguanodon; both then and later there were three or 

 four Plesiosaurs ; an Ichthyosaur ; another swimming Saurian, called 

 Pohjptychodon by Owen, nearly 50 feet long ; several Pterodactyls, one 

 of which, the P. giganteus, was 6 to 7 feet in the spread of its 

 wings ; a Mosasaur, 25 feet long (fig. 792) ; some Turtles. No true 



