40 Mr. Toulinin Smith on the Classification 
inination of the members of this family. My careful attention 
has therefore been directed to ascertaining the presence of that 
structure under every various mask of external form, and I have 
hitherto invariably found that presence accompanied by certain 
other characteristics, which would necessarily be present if the 
affinities which I have already attempted to show are those of 
the Ventriculidfe be the true ones. Without full confidence in 
the Law of Unity as a sure guide, I cannot conceive of any pro- 
gress being made in any scientific investigation. I have not 
found that guide to fail me yet in the present investigation, and 
am therefore content to take it as the basis of such exposition as 
I am now able to give of the genera and species of the family 
Ventriculid^. 
Proceeding therefore on this basis, it may be stated generally, 
that all those fossils which are marked by a membranous struc- 
ture made up of cubic squares, with equally subtending octahedral 
fibre at the angles of union of those squares, belong to the family 
Ventriculid^e, and that all members of that family are marked by 
that structure. We shall find, it is true, thus associated forms 
externally most diverse*, and the alleged affinity of which would 
at first sight startle the inquirer ; which have indeed hitherto had 
places the most different assigned to them : but I shall be able to 
show that other and most interesting Unities prevail through all 
these various forms in addition to that structural one ; and these 
diversities will thus become only another useful addition to the 
often repeated but too often neglected lesson, that no guide is 
more fallacious than likeness or unlikeness of mere external 
forint- 
A natural classification,” says Milne-Edwards, is nothing 
else than a description of the modifications, more or less import- 
ant, observed in the strueture of animals, and a specification of 
the differing degrees of likeness or unlikeness which the latter 
bear to each other J.” Nothing is easier than the multiplication 
of genera and species. But it is no slight task, though a most 
important one, to determine what are the material modifications 
on which distinction of genus should be founded ; what the ma- 
* On the other hand, I shall take a future opportunity of showing that 
forms externally bearing much resemblance to the Ventriculidae have in 
truth a very different structure and affinities. 
f Parkinson long ago remarked, that ^'if the figure of the fossil be as- 
sumed as the leading character of the species, substances, differing mate- 
rially in their structure, will be classed together in the same species ; and, 
on the other hand, if the species be formed on the external structure, we 
shall have under the same species substances differing widely in their 
forms.” Vol. ii. p. 128. It would have been well if Goldfuss and others 
had paid a little attention to these important truths. 
J Sur les Crisies, &c., p. 233. 
