45 



mination 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- 1 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 Ventriculidse 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.E. 



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 

 Ventriculidse, 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 

 formf. 



" A natural classification," says Milne-Edwards, " is nothing 

 else than a description of the modifications, more or less import- 

 ant, observed in the structure 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 Ventriculidse have in 

 truth a very different structure and affinities. 



t 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. 



} Sur les Crisies, &c., p. 233. 



