78 PROCEEDINGS OP THE GEOLOGICAL SOCIEXr. [DoC, 21, 



abundantly in these strata will serve to illustrate my meaning. These 

 are Gytlierea jyroxima, Desh., a species closely allied to 0. suheryci- 

 noicles, Desh., and Natica subdepressa, Morris. The former of these, 

 although not the Bracklesham Cytherea suberycinoides, looks strangely 

 like it. The latter, a Thanet-Sand species, has not, I believe, been 

 hitherto found above th.e base of the London Clay. The same re- 

 marks apply to several other species present less abundantly in 

 these sands — as, for instance, Gardita planicosta, Turritella sul- 

 cifera, T. imbricataria, and others, all "well-known species of the 

 Middle Eocene, while, on the other hand, Cardium Laytoni, Morris, 

 and Cytherea orbicularis, if not strictly Thanet-Sand species, range 

 elsewhere only into the lowest of the London-Clay strata. 



By taking a Darwinian view of the matter, one may perhaps get 

 over the difficulty of the seeming mixture of species ; and this 

 view is probably correct, for on close examination it is evident that 

 the common Cytherea of the Portsmouth sands is not quite the C. 

 suberycinoides of the Middle Eocene, although probably its pre- 

 decessor, in the same way as its companion shell, Natica subde- 

 pressa, var., may be the descendant of the N. subdepressa of the 

 Thanet Sands. 



The condition in which the fossils occur in the sands with Lingula 

 is also worthy of notice. In the lower portion of these sands, as also 

 in the underlying " Sands with Bentalium," the shells occur spa- 

 ringly, mostly with their valves united, and frequently in their natural 

 position. Towards the top of the " Sands with Lingula," where the 

 shells lie crowded together in layers or patches, there is clear evi- 

 dence of drifting. The univalves are often slightly worn. The 

 bivalves appear frequently with their valves disunited, and with the 

 concavity of the valve turned downwards, as is so constantly the 

 case between the tide-marks on a flat sandy shore. Fragments of 

 wood or carbonaceous matter, in minute quantities, are constantly 

 present in and near the layers of shells *. There can be no doubt 

 that the " Sands with Lingula " were either accumulated in shallow 

 water, or as a littoral deposit. 



(4). Clays with Cyprina. — This group includes the following 

 beds : — 



H. Brownish clay, with rounded black flint pebbles 8 inches. 



J. Stiff greyish clay with Septaria 30 feet. 



K. Brown sandy clay, with thin lines of sand 16 to 26 feet. 



Fossils are far from abundant in this group. The altered condition, 

 in all probability a deepening of the sea-bed, resulting in the intro- 

 duction of the layer of pebbles above mentioned, and the abrupt 

 change from sands to stiff clays, is seen to have been accompanied 

 by a marked change of fauna. Of the numerous species of MoUusca 

 occurring in the " Sands with Lingula,^' many of which abound to 

 within an inch of the zone of pebbles, a few species only reappear 

 at a higher level in the series. 



* Some of the blocks of stone, on being split in the line of bedding, are 

 seen to be crowded with carbonaceous markings, as of minute leaflets or seed- 

 vessels. 



