132 BRITISH PALEOZOIC ASTEROZOA. 



The ambulacralia are stout, oblong, closely-fitting plates. Stout -l-shaped ridges 

 run across their oral faces and serve to separate the depressions for the tube-feet. 

 Down the middle of the groove runs the ambulacral channel for the radial water- 

 vascular and blood-vessels. The concave depressions between each of the opposing 

 pairs of ossicles doubtless served for the attachment of the ventral cross-muscles. 

 Primitive ambulacralia very much of this type are found in several of the 

 Asterozoan branches and have already been described for Promopal deader (Text- 

 fig. 51a, p. 95). An important character in the ambulacralia of Uraxterella is 

 the position of the depression for the muscles connecting the ambulacralia with 

 the adambulacralia. The photograph given (PI. VIII, fig. 2) shows that these 

 articular depressions are situated well on the proximal face of the ambulacralia 

 and shaped so that their median plane dips sharply towards the mouth. The effect 

 is to make the oral surfaces of the adambulacralia also tip towards the mouth. 



Text-pig. 83. — Plan of the model of Urasterella t'iraivensis photographed PI. VIII, fig. 1. Ad., adambula- 

 cralia ; /)., depression for the muscle between ambulacral and adambulacral ; RL, ambulacral ridge, x in. 



The description of the ambulacralia of U. pulchella and U. medusa given by 

 Hudson (93, pp. 123-127) agrees on the whole with this. He does not seem to 

 have obtained clear oral views of the ambulacralia, but he has obtained, what I 

 have not, excellent apical views. He shows that there is no room between the 

 plates for the passage of ampullae. This is shown in one of his photographs 

 reproduced here, PI. VIII, fig. 7. He also shows that the imbrication of the 

 ambulacralia varies according to their position, that is, a few proximal ossicles 

 slope towards the mouth, but the majority slope away from it (op. cit., p. 123). In 

 order to make this clear 1 have given a simple diagram (Text-fig. 84). 



Schuchert (85, p. 172) describes the ambulacralia as being alternate. With 

 respect to this Hudson remarks (op. cit., p. 127) : " The arrangement of the flooring- 

 plates in U. medusa is for the greater part an alternate one. Those oppositely placed 

 being those of the peristomial ring and a few following them ; arm a has nine 

 plates oppositely placed, arm b but two pairs, and arm c apparently but one pair. 

 In U. pulchella, however, in both holotype and plesiotype, the arrangement so far 

 as seen is an opposite one. Only confusion can come from persisting in using this 



