Mr. Lyell on the relative ages of Crag. 409 



Norwich ; and this association of remains of the mastodon and 

 horse, both in Norfolk and on the continents of Europe and America, 

 Mr. Owen considers as a subject not without interest. 



Mr. Lyell examined also the crag north of Norwich at several 

 pits between that city and Horstead, and ascertained that it was 

 of the same kind, resting upon chalk, and overlaid by gravel. He 

 found in it Fusus striatus, Turritella terebra, Cerithium punctatum, 

 Pectunculus variabilis, Tellina obliqua, T. calcarea, Cardium edule, 

 and Cyprina vulgaris. 



3. On the third point, the relative antiquity of the Norwich to 

 the Suffolk 'crag, and the degree of resemblance of its shells to those 

 of existing series, the memoir contains much very valuable inform- 

 ation. The author acknowledges his obligations for assistance 

 during his researches, to Mr. J. B. Wigham, who has nearly 

 doubled the number of Norwich species of testacea ; to Mr. Searles 

 Wood, who gave Mr. Lyell free access to his fine collection of crag 

 fossils ; and to Mr. G. Sowerby, for the careful comparison and de- 

 termination of the recent species ; he also acknowledges the aid 

 afforded him by Mr. Fitch of Norwich, and Capt. Alexander of 

 Southwold. 



The total number of species in the Norwich crag, rejecting those 

 varieties formerly considered to be distinct species, is 111 , of which 

 19 belong to land or freshwater genera. This comparatively small 

 number of species, whether compared with the testacea of the Bri- 

 tish seas or the Fauna of the Suffolk crag, and not due to want of 

 activity on the part of collectors, or a paucity of specimens, Mr. 

 Lyell explained by showing, that in seas, the water of which is 

 only brackish, as that of the Baltic, or any great estuary, species are 

 far less numerous than in the salt sea, latitude, climate, and other 

 conditions being the same. A similar scarcity of species exists also 

 in the fluvio- marine deposits along the Rhine, between Basle and 

 Mayence. 



Of the 92 marine shells of the Norwich crag, Mr. Wood has 

 recognised 73 species found in the red crag, and therefore it might 

 he inferred that the two formations are nearly of the same age ; but 

 on applying the test of the proportions of recent species, Mr. Lyell 

 ascertained that the Norwich crag, both with respect to the marine 

 and the freshwater shells, contains between 50 and 60 per cent., 

 whereas in the red crag there are only 30 per cent., and in the 

 coralhne but 19. 



Mr. Charlesworth had previously implied that the Norwich beds 

 were the most recent, by stating his belief that shells had been 

 washed out of the red crag into the Norwich ; and both he and 

 Mr. S. Wood had recognised in the Norwich beds a nearer approach 

 to the existing British Fauna. 



The only known freshwater testacea of the red crag of Suffolk 

 were collected by Mr. S. Wood at Sutton, and consist of three 

 specimens of Auricula myosetis and one of the variety of Planorbis 

 marginatus, with a slightly prominent keel: both of these shells occur 

 in the Norwich crag. Among the other freshwater species of the 



