CLASSIFICATION 113 



tion of the general type relates to the construction of the posterior interradius, 

 upon which two well-defined divisions may be recognized : ( i ) the strong anal 

 side, in which the anal plates, when present, are partly or wholly incorporated 

 in the calyx wall; (2) the weak anal side, in which the anal plates are sepa- 

 rated from adjoining brachials at one or both sides by a calcareous skin, and 

 tend to form a flexible series supporting the anal tube, which is a mere exten- 

 sion of the pliant perisome of the tegmen. The second of these structures is 

 analogous to that found in the larval stage of the living crinoids ; it is the most 

 generalized, appearing in the Ordovician and ending with the culmination of 

 the group at the close of the Lower Carboniferous. The first is more special- 

 ized, and so far as known began in the Silurian, continuing also to the end of 

 the group; within it modifications in the size and position of the infrabasals, 

 and in the general habitus of the calyx, afford ground for family divisions 

 which shade into one another to some extent. 



Among the genera of each section may be observed a migration of the 

 radianal, from, a primitive position directly underneath the right posterior 

 radial (as if the radial were transversely bisected) in the older formations 

 (Ordovician and some Silurian), to an oblique position at the lower left of the 

 radial (some Silurian and Devonian), and then to an upper oblique position or 

 complete elimination in the Carboniferous. Modifications in the number of 

 primibrachs, by increase from two to three or more, were also to some extent 

 coincident with these changes. The structure of the base remains remarkably 

 constant, the proximal circlet consisting of three unequal infrabasals, which 

 are exceptionally fused into an undivided discoidal plate, and in certain genera 

 tend to disappear by atrophy. 



As a result of the foregoing considerations the Crinoidea Flexibilia fall 

 into two general groups, based upon the construction of the posterior inter- 

 radius : A, represented by such forms as Sagenocrinns; and B, represented by 

 Taxocrinus. These, which may for convenience be said to represent the strong 

 and the weak calyx respectively, will take rank as suborders, characterized as 

 follows : 



A. With a strong calyx: Sagenocrinoidea. This group includes all 

 forms in which the posterior interradius either is not differentiated, or is more 

 or less occupied by anal plates usually not in the form of a tube, the first plate 

 of which is to any degree incorporated in the calyx; that is to say, is suturally 

 united with adjacent brachials for at least part of its height on both sides. A 

 genus like Sagenocrinus, in which the anal interradius is an enlarged copy of 

 the others and forms a continuation of the solid calyx wall, will of course be a 

 perfectly evident typical example of this structure. In a general way this 

 group stands for a calyx with solid wall all around, as opposed to one in which 



