W. H. Sudleston — On the Yorkshire Oolites. 151 



Tlie seventh zone. — In the Lowee Calcareous Grit, except in 

 the very lowest portions, fossils begin to occur more scattered about, 

 but there is a calciferous or cherty band in Cayton Bay which 

 contains well-preserved specimens of the ammonites mentioned in 

 the table, together with bivalves and a very few Gasteropoda. 



This completes the description of the zones or horizons from which 

 the fossils forming the subject of Memoir No. 2 have been derived. 



It must be conceded that the condition of many of these is by 

 no means favourable to close determination. Moreover, differences 

 of preservation due to the varied nature of the matrix have, I feel 

 sure, exercised considerable influence on the external appearance of 

 many of these shells, so that when a "species" wears a totally 

 different dress, its identity is liable to be lost. Added to this is the 

 further difficulty that some " species " have been founded on casts, 

 without much regard to what might be the chances of the shell 

 itself having been described by another author from a distant locality 

 under a totally different aspect. 



These and many other circumstances have tended to make the 

 task of describing the Gasteropoda of the Lower beds a far more 

 serious undertaking than was the case with the better preserved 

 Corallian fossils described in the first Memoir, The usual difficulty 

 of nomenclature is of course the most grievous, and one begins to 

 recognize the fact that a fossil name is often little more than a label 

 which enables the collector or curator to arrange his specimens in 

 something like order. If there happens to be any mistake, such a 

 mistake is likely to be handed down to posterity for an indefinite 

 period. For example, Natica cincta, Phil., an Inferior Oolite fossil, 

 was accidentally classed by Phillips in the Coralline Oolite, and has 

 been so quoted by most palasontologists. One can easily believe 

 that for a hundred years to come it will be quoted in some places as 

 a Corallian fossil. It is easier to make a new rope than to straighten 

 one that has got a kink. 



With such materials and under such conditions it is not to be 

 expected that, in selecting names for a fossil, those most appropriate 

 in a biological sense should always be available. Indeed, it is not 

 seldom more difficult to locate a specimen generically than specifi- 

 cally — witness the perpetual shifting from one genus to another of 

 certain familiar species. Provisionally, in some of these cases, it 

 might save trouble to adopt the genus of the founder of the species. 



In concluding these introductory remarks, I would observe that 

 Gasteropoda bear an extremely small proportion to other Mollusca in 

 the beds under consideration. A collector may amass a considerable 

 number of specimens of all sorts before he obtains any univalves fit 

 for figuring. Nor are they so useful in determining horizons as the 

 Cephalopoda, for instance, since they seem to have been more 

 influenced by "conditions," and are thus less reliable as indicators 

 of time. 



My best thanks are due to the authorities at Cambridge, York, the 

 British Museum, and Jermyn Street for the facilities and assistance 

 afforded. [To be continued in our next Number.) 



