AMBULACRA OF THE KECEKT DIADEMATID^. Il3 



Plate 1 is a low demi, of the same shape as the corresponding 

 plate in the compound plate above, but the part of the plate 

 toward the interradium is very low in consequence of the height 

 of plate 2 at that part. Plate 2 is of the same shape as tha,t 

 already mentioned in considering the other compound plate, but 

 the pair of pores is near the interradial edge, and so the plate is 

 not low there. Towards the median line the plate expands, aud 

 it forms the whole of the compound plate there. Plate 3 is a 

 demi plate, but the pair of pores is in the normal position, and is 

 nearer the median line than the pair of plate 2. 



The alternation of the position of the pair^ of the demi plates 

 and of the central primary is unexampled. The relative position 

 of the plates is not that of Astroipyga, and it has a kind of 

 similitude to that seen in Gcelopleiorus. Certainly the arrange- 

 ment is generic and most distinctive. 



VIII. G-enus Aspidodtabema, A. Agassiz, 1879. 



Having had the opportunity of examining specimens of 

 A. microtiiherculatum, A. Agass., at the British Museum, I can 

 testify to the ability and truth of the description given by A. 

 Agassiz of this interesting dweller in the deep ocean (Eeport 

 on the ' Challenger ' Echinoidea, p. 6i, pi. viii. figs. 10-16). 



One of the specimens is fractured, and I have been able to 

 compare the drawing of A. Agassiz of the outside of the ambu- 

 lacra with the structure on the inside of the test. There is but 

 little difference, and, as might be expected, the adoral pore of 

 each ambulacral plate is on the transverse suture between two of 

 the plates. The plates are not in triplets, are narrow, and a 

 little broader than high. Each plate is independent of that 

 placed above and below, and in fact the ambulacra resemble 

 those of the Cidaridse. Of course the simple plates of Aspido- 

 diaclema are the analogues of the newest or last-formed plates of 

 the Diadematidse, close to the radial plate, and of those of the 

 Ccelopleuridae and TemnopleuridEe in similar positions. 



DESCJRIPTION OF PLATE V. 



Fig. 1. Biadema setosum, Gray. An ambulacral compound triplet plate, seen 

 from within, after the application of benzole. Magnifiecl. 



2. A compound plate between the ambitus and the radial plate, from within. 



Magnified. 



3. An ambulacral compound plate near the ambitus, external view. Mag- 



nified. The sutural lines are visible after the application of benzole 

 when the light is cast through the test. 

 LINN. JOUEN. — ZOOLOGY, YOL. XIX. 9 



