250 
nated by Phillips " arms of Cyathocrinus (Greol. Yorkshire, 
Pt. IL, pi. 3, fig. 38) to this genus. The figure calls to 
mind such forms as Z. Troostiantis, Meek and Worthen 
(Pal. Illinois, pi. 16, fig. 2), and Z. elegans, Hall (Geol. Surv. 
Iowa, vol. i., pt. 2, palaeont., pi. 9, figs. 1 and 2). 
IV. — Gomparative Review of the Groups above-mentioned. 
An examination of the resemblances and differences 
presented by these genera will find place more appropriately 
here than at the close of the description of each type 
separately; since the afiinities which exist between allied 
groups render such a review more intelligible and interesting 
when the forms in question are considered together and 
placed side by side for comparison. 
In the first instance it should be borne in mind that 
Naturalists have been content, hitherto, in the classification 
and grouping of Crinoids, to base their determinations upon 
differences in the arrangement and relative proportions of 
those plates alone which enter into the composition of the 
calyx or body-w^all of the crinoid; and have neglected 
almost entirely any consideration of the general morphology, 
or physiognomy even, of the forms to which they have 
assigned type-characters. 
Although it be true that through the action of co-relation 
every portion of an organism will become altered more or 
less, according as the surrounding conditions of life induce 
changes in the organs upon which these parts stand in 
immediate dependence ; yet it is self-evident that a classifi- 
cation founded upon a general comparison of type with 
type, taken as ivholes, is more likely to be a natural and 
philosophical grouping, than one which is based solely upon 
the variations of an isolated feature, however great its impor- 
tance per se may be. For although slight differences may- 
be maintained with a persistency which points to organic 
