Vol. 54.] AND THE COEALLINE CRAG. 313 



such countless profusion, are equally rare at Lenham and in the 

 Miocene deposits of Belgium. 



The fauna of the Isocardia cor-beda of Belgium, which resembles 

 so closely that of the Coralline Crag, presents, on the contrary, no 

 such marked affinities with the Miocene and the Italian Pliocene 

 deposits. 



While thus the Lenham fauna contains at least 13 (out of 

 61) characteristic Miocene or Italian Pliocene species, unknown 

 or rare in any North Sea deposit later than the Miocene, not a 

 single mollusc (the undescribed forms perhaps excepted) has been 

 met with at Lenham which has not also been found in beds as 

 old as the Coralline Crag. The moUusca occurring at Lenham, but 

 not in the latter, are therefore almost entirely of an older rather 

 than of a newer type, 10 out of the 15 before mentioned being 

 extinct and 3 southern. 



The evidence of the Lenham polyzoa is not of great value, 2 species 

 only being known to Mr. Reid, but, so far as it goes, it points 

 in the same direction. One of these, Fascicularia aurantium, is 

 extinct, and the other, Cupidaria canariensis, a form still existing, 

 ranges no farther north than Madeira or the Canaries. The poly- 

 zoan fauna of the Coralline Crag, on the contrary, includes many 

 species which are found in British seas. 



Mr. Reid indeed insists that the fossils from Lenham present a 

 more decidedly southern (but therefore, I suggest, older) facies 

 than do those of any other of the recognized Pliocene deposits 

 of the Anglo-Belgian area. The present distribution of moUusca 

 in the British seas seems to be largely due to tidal currents which 

 carry forward the free-swimming larvae as far as their influence 

 extends. As will be seen hereafter, the tidal currents which 

 reached the Coralline Crag area probably came, not from the 

 north, but from the south — that is, from the direction of Lenham, 

 It seems, therefore, to me more reasonable to suppose that the 

 southern and Miocene forms found at Lenham, but not in the 

 Coralline Crag, had died out in these latitudes previously to the 

 deposition of the latter, than that two faunas differing so consider- 

 ably one from the other should have co-existed in the same basin, 

 and under similar conditions as to current- action, within 60 miles 

 of each other. ISTo such difference in geographical distribution as 

 this is known to occur in British seas at the present day. 



The question of the origin of the ' boxstones ' found in the 

 nodule-beds at the base of the Coralline and Eed Crags may be 

 conveniently discussed at this point. They are composed of fino 

 ferruginous material, not unlike that of some of the Diestien or the 

 Lenham sandstones, and are rounded and waterworn, resembling 

 in shape the beach-pebbles of flint now found in places on the 

 Norfolk coast. Mr. Reid believes that they have been derived from 

 a single horizon, which is not of Miocene age,^ and with this con- 



^ Mem. Geol. Surv. 1890, * Pliocene Deposits of Britain,' p. 12, etc. See 

 also Eay Lankester, Quart. Journ. Geol. Soc. vol. xxvi (1870) p. 500. 



