Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/184

 abundantly in these strata will serve to illustrate my meaning. These are Cytherea proxima., Desh., a species closely allied to C. suberycinoides, 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 the base of the London Clay. The same remarks apply to several other species present less abundantly in these sands — as, for instance, Cardita planicosta, Turritella sulcifera, 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 predecessor, in the same way as its companion shell, Natica subdepressa, 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 Dentalium," the shells occur sparingly, 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 evidence 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 15 to 25 feet.

Fossils are far from abundant in this group. The altered condition, in all probability a deepening of the sea-bed, resulting in the introduction 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 Mollusca 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.

seen to be crowded with carbonaceous markings, as of minute leaflets or seed- vessels.
 * Some of the blocks of stone, on being split in the line of bedding, are