358 SPECIAL GEOLOGY. 



mollusca, about twenty of which are land and fresh-water 

 shells. "With these are found remains of fishes, Platax, Mylio- 

 bates, and mammalian remains of mastodon, elephant, horse, 

 pig, &c., and also bones of birds. 



The red crag is a shelly sand of a deep ferruginous colour 

 containing an abundance of marine shells, many of which 

 are much rolled or water-worn, the layers of which are 

 obliquely disposed from some -irregular movement during 

 their deposition. Between two and three hundred species of 

 mollusca are found in this bed, of which figs. 252, 253, 254, 

 255, are very characteristic. 



The coralline crag consists of a loose mass of shells and 

 corals, imbedded in calcareous sand ; or it is compact, and 

 forms flaggy beds of limestone, with bands of greenish marl. 

 Some of the harder parts are employed as a building material; 

 its fossil remains consist of spongidae, corals, and echino- 

 derms, with about four hundred species of mollusca, of which 

 figs. 256 and 257 are characteristic. It has been ascertained 

 that the coralline crag was partially consolidated before the 

 deposition of the red crag, as its surface has been denuded, 

 and perforated with Pkolades, w r hich lived in the sea that 

 deposited the latter.* 



A tooth of a gigantic Carcharias (fig. 258), found in the 

 red crag of Suffolk, and described by Mr. Charlesworth,f 

 proves that enormous sharks inhabited the shores of our 

 island during the miocene stage. 



Sir Charles Lyell gives the following table, illustrative of 

 the numerical proportion of recent and fossil species of mol- 

 lusca, from the stages of the English tertiary period. 



PEIUODS. LOCALITY. Percentage. 



P L ,OCE NE .{^ e ^ e ^ e8 >toiao. . . 40 



NEWER PLIOCENE Marine, near Glasgow 1 85 to 90 . . .160 



.4*0 



T-, f London, Hampshire 1 , 



EOCENE . . . | ^ j 1 or 2 ... 400 



* Tennant's British Fossils. 

 Magazine of Natural History, new series, vol. i. p. 226. 



