36 NEW YORK STATE MUSEUM 



ossicles, relative decrease in the size of the plates, introduction of 

 many new series of accessory pieces, absorption and removal of 

 others, with a marked general tendency to break up the stiff and 

 ponderous inherited skeleton into one of small pieces, thus afford- 

 ing greater flexibility and greater podial strength through the end- 

 less duplication of ambulacral parts." 



•In the half-grown (adolescent) individuals (see pi. 9, fig. 2) 

 the most characteristic features are, on both the actinal and 

 abactinal sides, the fairly uniform size of the columns of ossicles 

 at corresponding distances from the center, while in the mature, 

 and still more so in the gerontic stage, great differences develop 

 in the size of the adambulacral and inframarginal and a less marked 

 differentiation in that of the radial and supramarginal plates of 

 the arms. The primary plates of the disk remain entirely behind, 

 as shown above, and sink almost to the size of accessory granules. 

 The proximal radials and proximal axillary (proximal supra- 

 marginals) plates are all circular, knoblike in shape and remain so 

 throughout, although figured as oval bodies by Hall and later 

 authors. They contrast thereby with the following oval plates 

 in the columns. Further, all the plates are much more convex 

 or thick in the younger stages than in the mature and gerontic and 

 the arms correspondingly higher and thicker. This is also a 

 character clearly shown in the primitive Hudsonaster and of prob- 

 able phylogenetic importance. 



On the actinal side the axillary interbrachial plates have retained 

 their original circular shape, as. is well shown in Hall's figure (see- 

 also plate 8, figure 2 and plate 9, figure 3). The inframarginals are 

 throughout of convex subquadratic instead of long rectangular 

 shape, as in the later growth stages ; where, however, this stage is 

 retained near the ends of the arms. The plates of the oral armature 

 are apparently as in the older individuals. 



The smallest specimen available (radius 4 mm) shows a still 

 greater uniformity in the size of the plates. 



A few abnormal specimens deserve separate notice. Two of 

 these have but four, another has six arms. The former show all 

 plates in tetrameral arrangement, and one ray has therefore been 

 completely suppressed ; in the latter, however, which is seen from 

 the actinal side, only five plates of the oral armature and a corre- 

 sponding number of axillaries can be counted, and two arms are 

 irregularly coalescent at their bases as if they originated from one 

 arm that was torn off, a ca*se sometimes observed in recent 

 specimens. 



