48 BRITISH PAL.EOZOJC ASTEROZOA. 



I'cadily recognisable as Ijelougiiig to tlie class, l)ut at the same time presenting 

 characteristics which enable them to be placed in special families distinctive of 

 ]\ilteozoic times. 



The various species of Ai^phlosoma Avere shown to have a structure which 

 <liffered considerably from tliat of an Asteroid or a Recent Ophiuroid. Indeed the 

 research suggested to Schondoi'f that the great majority of the Palaeozoic forms 

 wliicli had previously been regai'ded as Ophiuroidea, and several which had been 

 classified with the Asteroidea, really belonged to a third new class, the Anluroidea. 



The characters of the Anluroidea (Text-fig. 23, p. 23) are as follows : 



(1) The radial water- vascular vessel runs in a canal wdiich is completely 

 enclosed on all sides by ambulacralia which are approximately half-cylinders. 



(2) Side branches of the vessel enter an open broad arm-groove through short 

 canals which (</) sometimes penetrate the substance of the ambulacralia, [b) at 

 others run between every two ambulacralia. 



(3) The ambidacralia are never fused with each other ; at times they are 

 opposite, at other times they are alternating. The adambulacralia are always 

 opposite to the ambulacralia. 



(4) Both the ambulacralia and adambulacralia send out processes which 

 meet one another. Both these series of ossicles are hollowed out on each side of 

 the processes. (The podia Avere contained in the cups formed in this manner.) 



(5) The ambulacralia are covered on their dorsal surface by spines and 

 granules. 



(6) The outer wall of the disc is concave (or convex ?), and may or may not 

 have marginalia. 



(7) The marginalia, if present, are confined to the disc, and are always sharply 

 separated from the arm l^y the adambulacralia. 



(8) The arm is bounded at the sides by the adambulacralia, and dorsally by 

 the ambulacralia. 



(0) There is a ventral arm-groove, which is broad and open. It is bounded 

 dorsally by the ambulacralia, and at the sides by the adambulacralia. 



(10) A typical madreporite lies ventrally in an interradius. 



(11) The forms are confined to the older Palgeozoic rocks. 



The three classes are considered of equal value in classification, and to be as 

 distinct among themselves as are the other classes of the Echinodermata (the 

 Echinoidea, Crinoidea, etc.). 



These characters can be analysed for the purposes of criticism as follows : 

 Frimitltje Characters. — (10) The ventral madreporite (see p. 37). 



(9) The broad open arm (arabulacral) groove bounded by the ambulacralia and 

 adambulacralia (see p. 23). 



(3) The unfused ambulacralia (see p. 42). The position of the ambulacralia 

 in respect to the adambulacralia (see p. 23). 



