120 SMITHSONIAN INSTITUTION 



The Ichthyocrinidae, with their infrabasals horizontal, withdrawn from 

 active participation in the formation of the calyx wall, and reduced to a mere 

 vertical plug entirely within and in nowise underlying the basals, represent the 

 nearest approach to a monocyclic condition that is found among the Flexibilia. 

 Having no connection with the exterior, and having lost their original function 

 as part of a cup enclosing the viscera, these plates are in an unstable condition 

 and exhibit a tendency to atrophy; and we therefore find in certain genera an 

 actual resorption of the infrabasals to a greater or less degree, extending in 

 some cases to complete disappearance. In Euryocrinus they are partly and in 

 Cleistocrinus wholly resorbed. In Amphicrinus the infrabasals are found both 

 partially and wholly resorbed in specimens of the same species (see PL XL, 

 figs, gb, 10). In at least two species of Ichthyocriiius the infrabasals are per- 

 sistently resorbed, the space which they should have occupied being sometimes 

 left open and well defined (PI. XXXV, figs. 2.b, 4b, 5). These instances are 

 comparable to the case of the Pentacrinidae, in which family infrabasals are 

 present in the genera Pentacrinus, Metacrinus, C omastrocrinus , Isocrinus and 

 Hypalocrinus, but always absent in Endoxocrinus. 



The Families of the Flexibilia 



A. Posterior interradius either not differentiated, or containing anal plates 



not usually formed into a tube, the first of which is incorporated in 

 the calyx by sutural union with adjacent brachials for at least part 

 of its height on both sides. Posterior basal, if differentiated, truncate 

 or angular, and suturally connected with succeeding anal plate. 



Suborder SAGENOCRINOIDEA. 



a. Infrabasals more or less erect, forming an essential part of the 



calyx wall. Crown usually short, rotund Lecanoceinidae. 



b. Infrabasals more or less recumbent, taking little part in the 



calyx wall. Crown usually elongate Sagenocrinidae. 



c. Infrabasals horizontal, not appearing exteriorly and taking no 



part in the calyx wall. Crown elongate or rotund. Rays 



widening upward to accommodate expansion of the calyx. . .Ichthyocrinidae. 



B. Posterior interradius always differentiated and occupied by anal plates 



in a tube-like series, none of which are suturally incorporated in 

 the calyx. Posterior basal rounded or indented distally and not 

 suturally connected with succeeding anal plate. Crown elongate. 



Suborder TAXOCRINOIDEA. 



A single family Taxocrinidae. 



