PLATANASTBR ORDOVICUS. 175 



very steep and the spines unnecessary. A widely open groove would necessitate, 

 on the contrary, the protection of the tube-feet by spines carried in the fashion 

 described. 



The adambulacralia are closely touching and slightly ridged at their points of 

 contact. It is difficult to imagine that the arms had much power of flexion. At 

 one point the ambulacralia and adambulacralia have been pushed away by post- 

 mortem disturbance, and the base of a super-ambulacral plate is seen. 



The infero-marginalia throughout the greater part of the length of the arms 

 are closely-fitting plates alternating with the adambulacralia. They possess 

 a large outer flat area ornamented with pustules, and a small inner sunk 

 unornamented area. At the extreme tip of the arm they become globular. 

 The two infero-marginalia which meet at the base of the arm are the largest 

 and are sickle-shaped (Text-fig. 114). Just within the disc are two large plates 



Px. 



Text-fig. 120. — Drawing of a portion of the apical surface of Platanaster ordovicus. Am., ambulacral ; 



I.M., infero-marginal ; Px., paxilla. x 12. 



which suggest, both by general appearance and ornament, that they are infero- 

 marginalia incorporated into the disc by fusion of the arms at the base. The 

 main axis of these plates is also sickle-shaped. 



Apical Surface (PI. XIV, fig. 1 ; Text-figs. 120, 121).— A side view of the 

 original impression of the form shows that the broad bordering plates are the 

 infero-marginalia. The apical surfaces of these are very similar to the oral faces 

 described already. The remaining portion of the apical surface was originally 

 covered with long slender paxillge in rows alternating with the infero-marginalia. 

 The paxilla? have fallen away from the central portions of the disc and exposed 

 the mouth-frame. The proximal ambulacralia distinctly diverge, forming descend- 

 ing V's along the radii (PL XIV, fig. 1) and ascending A's along the interradii. 

 Text-fig. 121 shows one of these ascending A's. The tip is occupied by the 

 highest portions of the mouth-angle plates ; the greater part of these plates lies 

 too low to be in view. The groove pointed to by the dotted line reaching to the 

 letter A is the groove for the water-vascular ring-canal. The canal was plainly 

 carried along the top of the ambulacralia constituting the V. The course of this 



