iS9v] 119 ["y^". 
tions I designatecl each genus as a series not as a genus, thus the 
genus Schlotheimia appears there ns the schlotheiniian series, 
Caloceras as the caloeeran series, and so on. This apphcation of 
explanatory terms seemed to me to make clearer the new mean- 
ing which it was proposed to give to the accepted taxonomic tei-nis, 
and furnished a set of parallel, popular designations which in cer. 
tain cases could be used in place of the scientific names of the 
genera. 
In this process the word "stock" was sul.'stituted for what 
would ordinarily liave l)een termed a sul)faniily. Thus, the 
plicatus and levis stocks distinguisiied hy the peculiarities of 
their young are really equivalent to two subfamilies in nu^ classi- 
fication, and their taxonomic equivalent is the radical stock which, 
however, consists of otdy one series or genus, Psiloceras. In so 
far as such terms maj^ be sometimes substituted for such highly 
artificial subdivisions of standard groups, their use seems to be 
unobjectionable and even an improvement, since they express 
different ideas of relation, and substitute natural for uniuitural 
associations. Nevertheless, they are essentially explanatory ternis 
which can be used side by side with ordinary aitificial terms of 
taxonomy and serve to help out the diagnoses. 
Mr. Blake proposes the term "mutation" or "form" for 
"species" and "lineage" for genus. It is hardly worth while to 
give a new meaning to "form." This is a very useful old word 
and without it as a staff to lean upon in general description one 
would often be at loss for an expression, but "mutation" might be 
very well used to explain a change taking place in a generic series 
which is of specific importance. The terms "variation" and 
"variety" have an already accepted meaning and are strongly 
entrenched in the literature of evolution as a})[)lied to groups of 
subspecific value. 
The substitution of "lineage" for series, which I have already 
used for the genus, is perhaps necessary. The term series, being 
suitable for general use, should not be restricted to any s[)ecial 
category of forms, and I shall hereafter adoi)t Professor Blake's 
substitute. 
In the Genesis of the Arietidae I have used the explanatory 
term "branch" where it seemed essential to unite two series of 
any one stock for descriptive purposes on account of their junction 
near the radical stock. This term has there l)een used in the same 
