54 THE CKINOIDEA CAMEEATA OF NOETH 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 Eiigeniacrinidas 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 fiYQ 

 plates, and the infrabasals either of fiNQ, 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 earhest dicyclic 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 Palaeozoic 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 ^yq 



