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 init. The 
work lies on the surface and relates wholly to the present, whereas the work of 
the palzontologist 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 paleontology, or 
rather that 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 
“oenus” and “ species”; whilst the still more artificial “family ” would be more 
difficult to outline the farther we go back in time. 
Divisions OF THE SuBJEOT. 
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 Nerinea, 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 
