158 BRITISH PALEOZOIC ASTEROZOA. 



the two series. They curve over so as to hide partially the ambulacra! groove. 

 The groove itself is arched over by stout ambulacralia. Small plates with a high 

 paxilla-shaft form the apical covering between the edges of the infero-marginalia. 

 The apical covering lies almost directly upon the ambulacralia, and there could 

 have been little if any space for the body-cavity. The ambulacralia, adambula- 

 cralia, and infero-marginalia are all firmly bound together, and only very rarely 

 can any displacement be observed. So firmly are they fixed that there is no 

 sign that the adambulacralia had any power to roll over and close the groove. 

 In oral view the adambulacralia form the entire margin of the arm. Each 

 adambulacral is distinctly swollen on both oral and outer surfaces. The swelling 

 is highest in the middle but extends almost to the margin, so that there is no 

 differentiated narrow ridge as in the Urasterellidae. It is ornamented by numerous 

 small irregularly arranged pustules which probably carried spines. None of the 

 spines remain attached, but there are many loose spines in the groove which have 

 doubtless fallen there from the adambulacralia. There is along each adambu- 

 lacral a narrow border devoid of ornament, doubtless the beginning of the differ- 

 entiation which resulted eventually in the confinement of spines to a central ridge. 



Further features which show the primitive condition of the ossicles of the 

 groove are (a) the prominent noses of the adambulacralia, (b) the " flooring-plate " 

 character of the ambulacralia. These latter are stout rectangular plates with a 

 high _l-shaped ridge. Some of their outer extremities may have been slightly cut 

 away to allow of the passage of ampullae, but it is extremely difficult to be quite 

 certain upon this point. The ambulacralia and adambulacralia are exactly opposite 

 to each other. The evidence for this is especially clear in D. 82. 



Displaced adambulacralia show that the articular faces have narrow circular 

 concavities for the insertion of the interadambulacral muscles. 



In side view the arm is seen to be steep-sided. Throughout its greater part 

 the infero-marginalia and adambulacralia alternate. Towards the base occurs a 

 modification which is illustrated in Text- fig. 101. The high odontophor in 

 growing upwards has carried the proximal infero-marginal with it. The neigh- 

 bouring infero-marginalia and adambulacralia are exactly opposite to each other. 

 All the infero-marginalia are swollen in an exactly similar manner to the adambu- 

 lacralia and possess the same type of ornament (D. 73). 



In apical view (PI. XII, fig. 1) the infero-marginalia only form a narrow border. 

 Between them stretches a broad expanse of spicula-like plates which are so similar 

 in appearance to those of Urasterella that one must regard them as paxilla?. In the 

 middle of the arm these paxillas are slightly larger than those nearer the edge, but 

 they are so irregularly arranged that one cannot separate them into radialia and 

 adradialia. The outer smaller spicules seem to fall more regularly into diagonal 

 rows of six. About three of these rows occupy the same length as does an infero- 

 marginal. Frequently the spicules have fallen away leaving the ambulacralia 



