The CalceocrinidcB. 391 



occasionally merged by anchylosis with the lateral radials, in which 

 case the lower portion of the anchylosed lateral radials extends 

 around and upon the posterior side of the calyx. 



The space between the posterior radials or the anchylosed lateral 

 radials is occupied by an azygous plate; and when the brachials belong- 

 ing to the posterior radials are present, they are crowded down between 

 them and the azygous, and help to form the arch around the consoli- 

 dated basal piece. 



Anterior radial either quadrilateral, usually elongate, and support- 

 ing the inferior side of anterior first brachial, or triangular and support- 

 ing the lateral radials, which in the latter case meet above it. 



The lateral radials are the largest plates of the calyx, and are of vari- 

 able size and shape, separated by an elongate anterior radial, or else 

 meeting over a triangular one, and always support the anterior brachial 

 between them. Their outer angles, and in case of an anchylosis with 

 the posterior radials, their outer sides, are bent around to the posterior 

 side of the calyx. Their upper and outer sides are inclined posteriorly, 

 and give rise to the lateral arms. 



When the arm-bearing function of only one of the posterior radials is 

 suppressed, the brachial lying above it gives support to the anal plates. 

 In case both posterior arms are wanting, both radials support the large 

 median anal plate, either directly or through the intervention of a T- 

 shaped azygous plate lying between them. 



Posterior radials, when present as such, lie in the angle formed by 

 the hinge-line between the consolidated basal piece and the lateral 

 radials. 



Azygous plate elongate and narrow, T-shaped or broad and low, 

 which latter form is probably due to anchylosis with another plate or 

 plates adjoining. 



The anal plate supports a large ventral tube, composed posteriorly of 

 a single series of curved quadrangular plates, " the anterior side being 

 composed of very small and delicate plates." ^ 



Anterior arm simple (?) or bifurcating once; if bifurcating, it may be 

 pinnulate. Lateral arms bifurcating at the second brachial, beyond 

 which point they may either increase by regular bifurcation or remain 

 simple, and give off pinnules; or they consist of a series of axillary 

 plates which are attached to' the anals by their posterior faces, and more 

 or less completely form an arch over the anal plates, the upper faces 

 being arm-bearing. 



Posterior arms when present resemble the lateral arms in structure. 



In the following genera, the order of their probable evolution 

 will be followed, and as that was in this case a kind of degener- 



^ Wachsmuth and Springer, "Revision of the Palaeocrinoidea," Pt. 

 III., p. 281. 



