166 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



supporting an anal plate. But among monocyclic forms the anal plate is 

 unrepresented in the Platycrinidae, 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 Hexacrinidae. The two groups thence- 

 forward flourished side by side to the middle of the St. Louis group, when 

 the Platycrinid^ became extinct ; while the HexacrinidaB and their offshoot, 

 the Acrocrinidge, 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 generahzations 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 Palaeozoic 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 Ichthyocrinidae, 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. Por 



