DIVISIONS OF SUBJECT. 3 



it to be final. Nay, the more exact it is the easier will it be to trace the links 

 which lead up to another group of forms. 



At the same time the malacologist must not be hard on the fossil- 

 conchologist because the latter cannot turn out such exact work as the stricter 

 requirements of modern biology seem to demand. The malacologist has a wide 

 grasp of space, but it is all on one plane as it were ; there is no depth in it. The 

 work lies on the surface and relates wholly to the present, whereas the work of 

 the palaeontologist lies in a series of perpetually receding planes, and bears the 

 same relation, metaphorically speaking, to the study of existing life that solid 

 geometry does to superficial. Of course it is admitted that palaeontology, or 

 rather tha£ branch of it which may be called mineral conchology, is to a certain 

 extent empirical in its methods, and these guesses at truth are not perhaps always 

 of the happiest kind ; still, it is more than probable that in the past there has not 

 always been such sharp divisions as are required by the logical definition of 

 " genus " and " species "; whilst the still more artificial " family " would be more 

 difficult to outline the farther we go back in time. 



Divisions of the Subject. 



The Gasteropoda are usually regarded as dwellers in shallow water, and hence 

 any notable accumulation of this class of shells would be looked upon as indicative 

 of seas of moderate depth at the time that such accumulations were made. In the 

 Jurassic rocks of England their distribution is very unequal ; considerable 

 thicknesses of rock are found to be almost devoid of Gasteropoda, and then again 

 a few feet of beds may contain large quantities of them. They are, in fact, less 

 sporadically distributed than the Brachiopoda or the Pelecypoda, and, on the whole 

 very much more difficult to procure in good condition. From their uncertain and 

 unequal distribution, and also from the fact that Gasteropoda have been greatly 

 influenced by the physical conditions that obtained during the period of deposition, 

 it is probable that they are of less value as indicators of horizons than the 

 Cephalopoda, the Ammonites especially. Moreover, with the exception of one or 

 two groups, such as Nerinaa, their mutations have been much less rapid, so that 

 forms have been more enduring, and thus sundry demoid types may be said to 

 pervade nearly all the beds. 



In dealing with our Jurassic Gasteropoda two alternative plans present them- 

 selves : (1) To carry each genus through from the lowest to the highest beds — 

 the biological plan adopted both by Goldfuss and d'Orbigny ; or, (2) to adopt the 

 stratigraphical plan, and to take no more than a series of beds for complete 

 description. The first method is the more philosophical of the two, but also the 



