182 BRITISH PALEOZOIC ASTEROZOA. 



the recent Ophiuroidea, by a portion of the apical surface being thrust round 

 into the oral inter-radii. In fact, we may say that Schuchertia is a fossil 

 representative of the "Asteroid" stage of the developing Ophiuroid. I am the 

 more confirmed in this view as I have evidence to be brought forward later 

 that there are Palaeozoic genera allied to Schuchertia which even more approximate 

 to undoubted Ophinroidea. 



Although the ground-plan of the oral surface of Schuchertia can be compared 

 with that of Hudsonaster, it must not be assumed, as Schuchert does (1915, 

 p. 194), that it is descended from that genus. In my opinion the apical surface 

 of the most primitive Asteroidea and that of Schuchertia differ so fundamentally 

 that one can only assume that the Ophiuroidea must have been differentiated from 

 the parent Asterozoan before the " Hudsonaster " structure was typically developed. 

 If this be true many features which are common to the recent Asteroidea and the 

 recent Ophiuroidea may not be due to the descent of the Ophiuroidea from the 

 Asteroidea as commonly assumed, but because of the inheritance of these characters 

 from a common ancestor. 



Sch/uchertia and its allies enable us to study these common ancestral characters 

 as well as the beginnings of typical characters of the Ophiuroidea. 



In dealing with these in the following pages, I have taken opportunity to 

 refer to the structure of recent forms, as many ambiguities in literature and 

 observation require clearing up. One point that emerges is, that there is not 

 such a fundamental difference between the Asteroid ambulacral and the Qpbiuroid 

 " vertebra " as was at one time imagined. Parallel evolution, probably due to 

 the action of similar muscles, leads to the independent production of similar 

 characters in several series. 



The Ambulacral Groove. — The ossicles of the ambulacral groove were un- 

 doubtedly laid down with all their essential peculiarities before the divergence 

 of the various Asterozoan stocks, for in all these stocks Ave can trace a common 

 primitive plan, based mainly on the support and protection of the tube-feet. 



I have already touched upon this in the Introductory Section of this Monograph, 

 but further points have emerged. Let us take, for example, the ambulacral groove 

 of Sch. ivenlocki (Text-fig. 124). The ossicles are shown in such a position that 

 one is looking straight across the groove on to the wall formed by the inner surface 

 of the adambulacralia. The nearest point to the observer is the exact radial line 

 of the arm. The aiubulacralia are of the flooring-plate type already described for 

 many primitive Asterozoa (Text-fig. 54^4, p. 95; PI. VIII, figs. 1, 2). Support is 

 given to the floor by a distinct overlap of successive plates. The floor is hollowed 

 in the middle (.1///. (.'//.) for the radial water-vascular vessel and accompanying 

 soft structures. Each plate has two ridges placed in the form of a — |. The cross- 

 ridge rises sharply from the body of the plate and provides one of the walls of the 

 cup for the reception of the retracted tube-feet ; the longitudinal ridge is parallel 



