MASTODON OF NORWICH CRAG. 637 



united by their ligament, in company with other littoral shells, such as 

 Mya armaria and Littorina rudis, and evidently not thrown up from 

 deep water. Yet the northern character of the Norwich Crag is not 

 fully shown by simply saying that it contains 12 Northern species now 

 no longer found in British seas, since several boreal shells which still 

 linger in the Scottish deeps do not abound there as they did in the lat- 

 ter days of the Crag Period. It is the predominance of certain genera 

 and species which satisfies the mind of a conchologist as to the Arctic 

 character of the Norwich Crag. In like manner, it is the presence of 

 such genera as Pyrula, Columbella, Terebra, Cassidaria, Pholadomya^ 

 Linyula, Discina, and others, which give a southern aspect to the Coral- 

 line Crag shells. 



In conclusion, it may be observed that the cold which had gone on 

 increasing from the time of the Coralline to that of the Norwich Crag 

 continued, though not perhaps without some oscillations of temperature, 

 to become more and more severe after the accumulation of the latter, 

 until it reached its maximum in what has been called the Glacial epoch. 

 The marine fauna of this last period contains, both in Ireland and Scot- 

 land, recent species of mollusca now living in Greenland and other seas 

 far north of the areas where we find their remains in a fossil state. 



It is not in reference to the two older formations above alluded to, 

 but when we attempt to classify the lacustrine and fluviatile deposits 

 (some contemporaneous with the marine Norwich Crag and others pos- 

 terior to it), that we encounter in the East and South of England the 

 greatest difficulty. When treating of the Newer Pliocene and drift 

 formation in the Valley of the Thames, I have acknowledged the per- 

 plexity in which this subject is still involved, and have hinted at the 

 causes of it (chap. xiii. pp. 152, 153). Every year, however, removes 

 some of this ambiguity ; for the true relative position of distinct sets of 

 superficial strata becomes more clearly understood, and the specific 

 characters of the fossil mammalia and shells better ascertained. In the 

 first place, the occurrence in the Norwich Crag of many marine shells 

 of Northern species, as before described, in company with land and 

 freshwater shells, and some mammalia of a more Southern character, 

 may possibly be explained by supposing the sea of the Norwich Crag 

 to have been opened towards the Pole, with islands interspersed, while 

 the land of the same period was continuous far to the South. In that 

 direction a Continent may have existed, from which rivers flowed north- 

 wards, in whose waters the hippopotamus and such shells as the Cyrena 

 consobrina flourished. 



The Mastodon found in the Red and Norwich Crag (p. 155, and fig. 

 135, p. 165) was till lately regarded as a Miocene or Falunian species; 

 and under this persuasion, calling it M. angustidens, on the authority of 

 Professor Owen, I suggested that its remains might have been washed 

 out of older strata into the Crag, just as we sometimes observe London 

 Clay and Chalk fossils occasionally introduced into the same deposit. 

 Many teeth of this Mastodon, together with numerous ear-bones of 



