TIIK STEM AMi OALYX, 



17 



at the distal ends of the ambulacra (PI. IV. figs. 13, IT.; PI. V. figs. 0, 8, 14, 29 ; 

 PI. XII. figs. 2, 3; PI. XV. fig. 0; PI. XVI. fig. Hi ; l'l. Will. fig. [2 ; I'l. XIX. 

 figs. 11-14). The general arrangement of these ridges is the same in all Blastoids, 

 and is perhaps best studied in Stephanocrinus, in which type they are very strongly 

 marked (Fig. III.). 



Fig. III. 



Diagram to show the positions of the ridges (1-8) on the basals and radials of Stephanocrinus 



angulatus (modified from Roemer). 



The lettering as in Fig. I. 



The small basal x is marked by two ridges (1 & 2) which rapidly diverge and 

 terminate respectively in the lips of radials A and B. There are three ridges on 

 basal y. The first (No. 3) passes on to radial B and meets at its lip the second ridge 

 from basal x. The next (No. 4), which is much more prominent, passes directly 

 upwards on to the middle line of radial C and ends in its lip ; while the third (No. 5) 

 terminates in the lip of radial D, where it meets a similar ridge (No. 6) from the next 

 basal, z (PI. XIX. figs. 11, 12). The median ridge of this basal (No. 7), also larger 

 than its fellows, passes directly on to radial E; while the eighth and last ridge joins 

 the first at the lip of radial A. Of these eight ridges four are stronger than the rest, 

 viz. the median ones on each of the large basals (Nos. 4 & 7), and the two diverging 

 ridges on the azygos basal (1 & 2). 



The same is the case in Tricaelocn'ntts, as seen in PI. XIX. fig. 14, which also shows 

 very clearly how the trimerous symmetry of the basals passes into the pentamerous 

 symmetry of the radials. In the first place, an additional side A-B is introduced by 



D 



