54 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
period of Pentacrinoid life they are transformed into the “rosette,” which 
closes the upper opening of the centro-dorsal cavity lodging the chambered 
organ. The Eugeniacrinide have no basals in the adult, and Carpenter sup- 
posed that they were anchylosed with the radials, while Zittel and Jaekel 
think they were enveloped by exuberant growth of the radials. 
In all dicyclic Crinoids the basals, without exception, consist of five 
plates, and the infrabasals either of five, three, or a single piece. The basals 
of monocyclic Crinoids vary in number from one plate to five. 
The proximal ring, whether consisting of one, two, three, or four pieces, 
whether basal or infrabasal, is divisible into five elementary plates; and the 
smaller number, where it exists, is produced, as we shall presently show, by 
anchylosis of two or more of the primary segments, accompanied by a more 
or less complete obliteration of the suture lines. 
The earliest dicyelie Crinoids had five infrabasals, and the first mono- 
cyclic ones five basals. Before the close of the Lower Silurian, there ap- 
peared two monocyclic genera with four basals, both having a special anal 
plate interposed between the radials. The quadripartite base reached its 
culmination in the Upper Silurian, and disappeared before the close of the 
Devonian. 
The earliest genera with a tripartite base occur in the Upper Silurian ; 
some of them have an anal plate, and others not. When that plate is 
represented, the basals are of equal size; when absent, two of the basals 
are equal, and the third about half smaller. The two forms continued to 
exist side by side to the end of the St. Louis group of the Carboniferous, 
when both became extinct. 
The bipartite base is restricted to the Carboniferous. It occurs from the 
Kinderhook group up to the Coal Measures, but is found only among genera 
with a large anal plate. 
It is evident from these observations that the number of basals was grad- 
ually reduced in Paleozoic times, and that in the Camerata the anal plate 
was introduced after the quadripartite base had made its appearance. It will 
now be shown that this diminution of number was the result of fusion of two 
or more of the five original plates, and that by the introduction of the anal 
plate the base underwent further modifications. The manner in which the 
modifications in the number of basals and infrabasals were effected, may be 
best understood by reference to the diagrams on Table A. 
Looking at these diagrams, the transmutation in the Camerata from five 
