THE CRINOIDEA 



97 



of the sinus, and immediately bifurcates, the groove forking with it. 

 The edges of the deltoids meet beneath the groove, but a space for the 

 mouth ("peristome") is left in the middle. This space, as well as 

 the whole groove, is covered by ambulacrals ; these often fuse into 

 a single plate on either side the groove, where it passes over the oral 

 surface of the theca, and form five plates, likewise often fused, over 

 the peristome (P). The anus is between posterior A and the adjacent 

 radial processes, and is closed by a valve of four to six small plates. 

 Certain pits in a similar position in other interradii possibly are 

 atrophied hy drospires (h). The ornament of the cup-plates is strongly 

 reminiscent of that in Eublastoidea ; but there are clearly marked 

 axial folds passing up from the basals to the arm-facets, perhaps due 

 to the greater development of the abactinal nerve-system in the 

 brachiate form. There were neither brachioles nor a lancet-plate. 

 Stephanocrinus undoubtedly belongs to the simplest and most 

 primitive group of the Crinoidea, and it is hard to believe that its 



FIG. II. 



Stephanocrinus angulatus. 1, 

 from anterior radius, x 2 diam. 

 2, from oral surface, x 4 diam 

 (from Brit. Mus. E6T15). As, anus 

 covered by plates ; Ax, axillair 

 from which are supposed to spring 

 two arm-rami ; Bi J , position fron 

 which these spring, between $ 

 spines formed by radial processes 

 broken off in 2, and showing (//) 

 supposed atrophied liydrospires ; 

 P, larjje covering -plates over tlie 

 peristome, which is surrounded by 

 the five orals or deltoids (0). 



As 



remarkable resemblances to Eublastoidea are merely homoplastic, 

 especially since the position of the small basal is not one which 

 usually occurs in other Crinoidea that have fused basals. 



However the crinoid or brachiate stage in the history of the 

 Pelmatozoa may have been reached, it will be useful to recapitu- 

 late here the common pelmatozoic characters as well as those 

 distinctive of the Crinoidea, as manifested in a normal crinoid of 

 simple structure. The specialisation of those characters will be 

 shown historically in the systematic part ; but since many 

 structures have been produced or modified in the same way more 

 than once, a general account of the processes may be given here. 

 We can speak more decidedly on questions of development and 

 internal anatomy in this class, since the differences between extinct 

 and recent genera are not such as to hinder interpretation. 



A normal Crinoid was thus described in 1821 by J. S. Miller, 

 the founder of the *class : " An animal with a round, oval, or 

 angular column, composed of numerous articulating joints, 

 supporting at its summit a series of plates or joints forming a 



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