96 PROF. p. M. DUNCAN OK THE ANATOMY OF THE 



of the classification of the group of Echinoidea with triple pairs 

 of pores ; but probably the readers of this communication, which 

 relates to the genera Diadema, EcMnothrix, CentrostepTianus, As- 

 tropyga, Micropya, and Aspidodiadema, will consider, with us, 

 tbat it should appear before the second part of the essay by 

 Mr. Percy Sladen and myself. 



II. G-enus Diadema, Gray. Diadema setosum, Gray. 



This well-known form usually breaks anywhere tlian at th.e 

 sutures which separate the plates of the test. The test is thin, 

 and is well covered with a semi-leathery tissue when dry, and 

 the plates have much connective tissue between them. 



Tests were carefully denuded by means of chloride of lime and 

 exposure to the weather, and there was no difficulty in then sepa- 

 rating any plates except the triplets of the compound ambulacral 

 plates. Eeagents, such as benzole, render the lines of the 

 sutures of those firmly united plates visible. 



The Amhulacra. — Taking a tubercle-bearing plate at the am- 

 bitus, the triplet of large pores, the large perforate and crenulate 

 tubercle, and a very small, yet perfect, tubercle close to the 

 median suture of the compound plate are readily noticed (Plate 

 V.fig. 3). 



The pairs of pores are in slight curves, and their direction 

 is rather oblique. The pairs are distant, their peripodia do not 

 touch, and there are often one or more minute tubercles, or gra- 

 nules, between them. 



The pores follow the rule regarding the position of triplets, 

 and the lowest or adoral pair is close to the actinal edge of the 

 compound plate and nearer to the median line of the ambulacrum 

 than the other pairs. The second pair is, as usual, the most 

 external of the three, and the first pair is more internal than 

 the second, and yet not so much so as the third pair. 



On examining an ambulacrum from within, it was found that 

 the arrangement of the triplets of the compound plates is not the 

 same as that seen in such TriplechinidsD as Strongylocentrotus, for 

 the second plate of every combination is the largest of the three, 

 and indeed it com^^oses mucVi of the compound plate. In the pori- 

 ferous part of a compound plate, the sutures between the first and 

 second, and between the second and third plates are not hori- 

 zontal, and as they approach the position of the tubercle they con- 

 verge slightly, so that the second or middle plate becomes nipped 



