166 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
supporting an anal plate. But among monocyclic forms the anal plate is 
unrepresented in the Platycrinidz, in which the base forms a pentagon. 
The introduction of the anal plate occurred in the Hamilton group, and 
produced the hexagonal base of the Hexacrinidez. The two groups thence- 
forward flourished side by side to the middle of the St. Louis group, when 
the Platycrinide became extinct; while the Hexacrinide and their offshoot, 
the Acrocrinidz, continued until the extinction of the Camerata. 
We have not attempted to construct a genealogical tree for the Crinoids, 
or a branch of one for the Camerata, because such representations are 
generally unsatisfactory, and in this case the tree would have to be con- 
structed too much upon imagination. Besides, our task is an humbler one. 
We have rather preferred to content ourselves in this respect with giving 
the general facts which our investigations seem to pretty well establish, and 
such interpretation of them as appears to us reasonably consistent therewith. 
Within these limits we have hoped that our generalizations may help to 
form a stable foundation upon which others may raise more ambitious 
structures. 
There is no doubt that the Crinoids, by reason of their great geological 
range, and capacity for individual variation due to their complicated struc- 
ture, offer one of the most inviting fields for demonstrating the principles of 
evolution, They afford a good illustration of the principle that individual 
development finds a parallel in a general way, in the phylogenetic history 
of the group. But while recognizing this truth, and confirming it by the 
many interesting proofs which our studies disclose, we must beware of ex- 
pecting to find lineal succession, or of assuming that a form found flourishing 
in any given epoch is necessarily more highly organized than those occur- 
ring in previous epochs. The Paleozoic Crinoids represent in a broad sense 
the larval stages of recent Crinoids; but there are many cases in which the 
tendency seems to have been one of retrogression instead of progression. 
Haplocrinus, with its closed pyramid of five orals, is probably the most primi- 
tive type found in our collections, and yet it is a Devonian genus. The 
great family of the Ichthyocrinide, whose oral condition is substantially on 
a par with that of many recent Crinoids, occurred abundantly in the Silurian. 
It is also impossible to tell, except perhaps in a very general way, which one 
of a number of variations marked the line of succession; or in other words 
which was for the time being the racial characteristic carrying all others 
along with it, even though many of them may seem more important. For 
