INTRODUCTION. 



21 



can have been little more than mere flooring plates to the ambulacral groove. 

 The adambulacralia have the nsual typical form. It is readily seen that the 

 depression for a tube-foot was formed by the inner margins of two consecutive 

 adambulacralia and by succeeding cross ridges of two ambulacralia. The ornament 

 on the adambulacralia probably consisted of sub-ambrdacral granules and slight 

 groove-spines. Eacli adambulacral is attached to one ambulacral. The articular 

 faces are subtriangular in form, and are slightly excavated for the insertion of the 

 vertical muscles (compare Text-fig. 10). 



Some advance in structure upon that already described is shown by the 

 Ordovician forms known as Palseaster raracfaci, Grregory, and I'rotopalasaster 

 narraioaiji, Hudson. These forms, in my opinion, belong to one species, referred 

 to here as Profopalxaster caradaci, of which there are several specimens in the 

 British Museum (Nat. Hist.) and the Museum of Practical Geology. One of 

 them possesses the ambulacralia preserved in the original calcite. From this 



Am. I. M. 





1 1 

 I 1 



1 1 rt-*'~V 



^ 





^^, 



L ._ 



. V - 



Ad. 



Text-fig. 19. — Apical view of extremity of arm of Pi-otopalaaster " narrMvayi" (after Hudson). Am . 

 ambulacral; Ad., adambulacral ; /. M., infero marginal. 



specimen v/e learn that these plates present the same oral view as do the 

 corresponding plates in Uranaster. 



The excellent photographs of Hudson^ give clear details of a specimen, which, 

 by a fortunate accident, has lost the dorsal plates of the disc, and so exposed 

 the ambulacral groove from above. One of these photographs is reproduced here 

 as Text-fig. 19. It is seen that the arm differs from that of Tirana ster in the 

 difPerentiation of stout marginalia. 



The figures also sliow that the ambulacralia are not exactly opposite to each 

 other but are slightly alternating, that there are no ambulacral pores, and that 



' Hudson (33), when lie described this form, thought that he was looking down on the ventral 

 (oral) surface. The ambulacral groove therefore appeared to him to be roofed over by " covering 

 plates," and he naturally regarded these as homologous with the " covering plates " of Cystids and 

 Crinoids (see p. 4). There can be no doubt that this interpretation is incorrect. As shown by 

 Eaymond (56), the fossil has its oral surface turned towards the rock, in which it is partially 

 embedded. The dorsal (apical) surface is the surface exposed to the observer. The great majority of 

 the dorsal plates of the disc are wanting, and in consequence the ambulacralia are exposed, arching 

 over the ambulacral groove in the usual Avay. 



