26 Mr. H. G. Seeley on the Potton Sands. 
long been in the habit of teaching that sands and sandstones are 
formed during upheaval, and therefore we may expect in them 
fossils denuded out of older strata; but we shall also almost 
inevitably have in the bed, of contemporary age, a mixture of 
the life of the preceding and of the succeeding periods*. 
VII. The author then says that the phosphatic casts of shells 
in their general aspect resemble those of the Kimmeridge and 
Oxford Clays. Had he taken the trouble to get a few of them 
named, he would have found that they were Portland species ; 
he would, moreover, have found that a large number of the casts 
are in sand cemented with phosphate of lime, and that species 
which are usually preserved as internal moulds occur with the 
shell preserved when contained in hard sandy nodules. 
VIII. Many of the Mollusca, as Mr.Walker has stated, occur 
with the shell replaced by oxide of iron. They are all in exactly 
the same state of preservation; but since our author imagined 
the bed to be Shanklin Sand, he selects a few which have affini- 
ties with Lower Greensand species, and discards the remainder 
as extraneous—a way out of a difficulty, as I imagine, hardly in 
accordance with scientific method. 
IX. Our author’s list of Mollusca, as far as it goes, is given 
with some approach to correctness. TI have seen no Terebratula, 
however, which corresponds with Prof. Morris’s celtica. But 
T’. celtica, T. prelonga, T. sella, T. tamarindus, and T. depressa, 
with some few others, will, I apprehend, hereafter be regarded 
as varieties of one species ; so that it is one of those shells which 
it would not be surprising to find. 
Pecten Robinaldinus is not a bad identification. But P. Ro- 
binaldinus, P. interstriatus, P. Galliennei, and several others are, 
I believe, only varieties of the elongatus of Lamarck, separated, 
like the Terebratule, because the series at the describer’s com- 
mand was too small to show the gradations of one form into 
another. 
Ostrea macroptera.—Although this is the name used by me 
for this fossil, as a variety of the O. frons of Parkinson, it is a 
form limited, so far as I know, to the Portland Rock, being 
usually attached by the whole of one valve, and having the. other 
valve nearly smooth—very unlike Sowerby’s typical O. macro- 
ptera. O. frons and O. gregaria are not to be separated as 
species. 
Pleurotomaria Deshayestt, though resembling that shell, is a 
variety of P. gigantea, intermediate between that species and 
P. rugata. 
* “The Laws which have determined the Distribution of Life and of 
Rocks.”’ Read before the Cambridge Philosophical Society, Nov. 12, 
1866. ‘ 
