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. I have seen no Terebratnla, 

 however, which corresponds with Prof. Morris's celfica. But 

 T. celtica, T.prcelonga, T. sella, T. tamarindus, and T. depressa, 

 with some few others, will, I apprehend, hex'eafter 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- 

 hinaldinus, P. interstriatus, P. Gallieiinei, and several others are, 

 I believe, only varieties of the elongatus of Lamarck, separated, 

 like the Terebratulse, because the series at the describer's com- 

 mand was too small to show the gradations of one form into 

 another. 



Ostrea macruptera. — 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. 0. frons and O. gregaria are not to be separated as 

 species. 



Pleurotomaria Deshayesii, though resembling that shell, is a 

 variety of P. gigantea, intermediate between that species and 

 P. rug at a. 



* " The Laws which have determined the Distribution of Life and of 

 Rocks." Read before the Cambridge Philosophical Societv, Nov. 12, 

 1866. 



