278 
ME. PRESTWICH ON THE GEOLOOT OF THE DEPOSITS 
for those of the Oise. I have given in the Appendix the results of this examination of 
the range of the quaternary species of the beds under review. 
High Levels. — The high-level gravel of St. Acheul has furnished a distinct and toler- 
ably complete list of Mollusca; but in England the evidence is more imperfect., Hoxne 
supplies but few species, and some uncertainty attaches to the level of Biddenham*. 
Out of a total of 109 land- and freshwater shells now inhabiting the South of England 
and North of France, 36 species have been found in the flint-implement-bearing high- 
level gravels ; and of the 12 freshwater genera 8 are represented in these old alluvia. 
There is a singular scarcity of Unionidse and Paludinidse. The Neritinidse are not 
present at all. The Limnseadse, of species inhabiting marshes and pools, on the contrary, 
abound, together with, in places, the fluviatile species of Ancylus ; the Helicidae are, 
Succinea excepted, poorly represented, compared to their numbers in the low-level gravels. 
A variety of species are common at St. Acheul, while at Montiers one species of Pupa 
(P. marginata ) occurs in thousands with but few other shells, and in the various other 
isolated patches of high-level gravels between Amiens and Abbeville, including the 
beds at Moulin Quignon, I have not been able to discover any traces of shells. They 
are equally rare in the high-level gravels of other valleys, except at a very few places. 
Combined with this scarcity, there is also an absence of such shells as would mark full 
deep rivers or lakes, whilst such as might be found in broad and shallow rivers, that, 
like many of those of Northern America at the present day, are flooded at one period 
of the year and nearly dry at others, occur at long intervals. In most of these northern 
rivers shells are also extremely scarce. Mr. Bell mentions an instance where he 
travelled along the banks of the river Magdalen for four weeks, during which the 
only shell he met with was a species of Limncea f . This scarcity of shells is common in 
most of the rivers in these high latitudes. Along such rivers, however, there are often 
quiet pools, where shells are more numerous. If to the limited fauna found under these 
circumstances we add the land-testacea carried down by freshets and by small tributary 
streams, the shells so brought together would form a collection very similar in character 
to that which we find in these post-pliocene deposits. 
At St. Acheul Mr. Godwin-Austeh called my attention to a large sandstone block 
having on its surface numerous rudely worm-shaped lines of concreted sand, bearing 
a very close resemblance to those made by the sand-covered gelatinous attached polyzoa so 
common in our clear stony-bedded streams, as a proof that it had lain in a running stream. 
Such specimens are not rare, and taken with the absence of mud or clay beds in these 
gravels, and the general character of the shells, lead also to the inference that the old 
rivers were usually clear and limpid. 
With respect to the species of Mollusca, they show, with few exceptions, a near 
* I give the list of shells I have found at Biddenham, together with the list of those found by Mr. Wyatt 
at Summer-house Hill in beds which certainly belong to the low-level series. I do not, however, feel sure that 
both Biddenham and Joinville, though distinctly above the low-levels, should not he placed on an intermediate 
level.— Feb. 1864. 
t Geological Survey of Canada, 1857 & 1858. 
