154 FLUVIO-HARLN'E NORWICH CRAG. [Cn. XIIL 



2000 mammoths' grinders.* Another portion of the same continuous 

 stratum has yielded at Bacton, Cromer, and other places on the coast, 

 the bones of a gigantic heaver {Trogontherium Cuvierii, Fischer), as well 

 as the ox, horse, and deer, and both species of rhinoceros, R. tichorhinns 

 and R. leptorhirius. 



In studying these and various other similar assemblages of fossils, we 

 have a good exemplification of the more rapid rate at which the mam- 

 miferous fauna, as compared to the testaceous, diverges from the recent type 

 when traced backwards in time. I have before hinted, that the longevity of 

 species in the class of warm-blooded quadrupeds is not so great as in that of 

 the mollusca, the latter having probably more capacity for enduring those 

 changes of climate and other external circumstances, and those revolutions 

 in the organic world, which in the course of ages occur on the earth's surface. 

 This phenomenon is by no means confined to Europe, for Mr. Darwin 

 found at Bahia Blanca, in South America, lat. 39° S., near the northern 

 confines of Patagonia, fossil remains of the extinct mammiferous genera 

 Megatherium, Megalonyx, Toxodon, and others, associated with shells, 

 almost all of species already ascertained to be still living in the contigu- 

 ous sea ;f the marine mollusca, as well as those of rivers, lakes, or the 

 land, having died out more slowly than the terrestrial mammalia. 



I alluded before (p. 131) to certain marine strata overlying till near 

 Glasgow, and at other points on the Clyde, in which the shells are for 

 the most part British, with an intermixture of some arctic species ; 

 while others, about a tenth of the whole, are supposed to be extinct. 

 This formation may also be called Newer Pliocene. 



Fluvio-marine crag of Norwich. — At several places within five miles 

 of Norwich, on both banks of the Yare, beds of sand, loam, and gravel, 

 provincially termed " crag," but of a very different age from the Suffolk 

 crag, occur, in which there is a mixture of marine, land, and freshwater 

 shells, with ichthyolites and bones of mammalia. It is clear that these 

 beds have been accumulated at the bottom of the sea near the mouth of a 

 river. They form" patches of variable thickness, resting on white chalk, 

 and are covered by a dense mass of stratified flint gravel. The surface of 

 the chalk is often perforated to the depth of several inches by the Pholas 

 crispata, each fossil shell still remaining at the bottom of its cylindrical 

 cavity, now filled up with loose sand which has fallen from the incumbent 

 crag. This species of Pholas still exists and drills the rocks between high 

 and low water on the British coast. The most common shells of these 

 strata, such as Fusus striatus, Turritella terebra, Cardium edule, and 

 Cyprina islandica, are now abundant in the British seas ; but with them 

 are some extinct species, such as JVucula Cobboldice (fig. 125) and Tel- 

 Una obi iqua (fig. 126). Jfatica helicoides (fig. 127) is an example of a 

 species formerly known only as fossil, but which has now been found living 

 in our seas. 



Among the accompanying bones of mammalia is the Mastodon 



* Woodward's Geology of Norfolk. f ZooL of Beagle, part 1, pp. 9, 111. 



