66 BRITISH PALEOZOIC ASTEROZOA. 



pustules are not due to the breaking up of growth-lines by radial stresses, but 

 they may very well have been tubercles bearing minute spines. Such spines, being 

 very loosely attached, would readily fall off after death, and would in any case escape 

 observation owing to their minute size. I have searched for them in the very small 

 amount of available material, but in vain, unless a tiny rod ('8 mm. X "25 mm.) in 

 the left anterior interradius of E. 15930 may possibly be one." The plates of the 

 primitive Starfishes show the same ornament. This has led them to be described 

 by some authors {e.g. Schuchert, 85, p. 31) as "granulated." It is probable, 

 however, that these granulations are not merely rugose outgrowths of calcite such 

 as are found in certain lineage stages of Cretaceous Starfishes (Spencer, 74, p. 103), 

 but are more comparable with the spine-bearing mammillations on the plates 

 of Echinoids. In certain cases, where the conditions of preservation were excep- 

 tionally favourable, the actual spines are preserved, as, e. g., on the infero- 



Text-fig. 33.— An infero-marginal of Coecaster bidhiferns, showing ornament, x 60. 



marginalia of Coecaster hulbifenis (Text-fig. 33). Schuchert has also figured 

 similar spine-bearing mammillations on apical ossicles of Promovalgeaster dijeri (85, 

 pi. 20, fig. 6). 



These observations show that the ornament on the primitive Asterozoa, the 

 Echinoidea, and the Edrioasteroidea, has many features in common. Whether 

 this is due to common ancestry or to parallel evolution in several groups, remains 

 as yet to be decided. The connection is not so clear in the later Asteroidea, 

 because in these forms the spine-pits are very frequently flush with the plates, and 

 not situate upon articulatory prominences. 



(C) Miscellaneous Details. 



The following details are of general importance : 



(1) Schuchert (85, p. 45) states that " in Huchonaster the prominent supra- 

 marginal plates of the dorsal side are placed decidedly inside of the inframarginals, 

 though the former clearly overlap the latter. This primitive position is retained 

 in many Palaeozoic genera, and apparently not before the Devonic do these two 

 columns of ossicles come to lie wholly upon one another, and then they together 

 margin the animals. It is apparently always a rare condition in the Paleeozoic, 

 but as the post-Devonic asterids are as yet little known, we can more accurately 



