AISTATOMT OF THE AMBTJLACEA OF THE RECENT DIADEMATID^. 95 



diverse, and is constant in its diversity ; and there are other 

 differences which, though eluding attention at first, are really 

 very marked. 



DESCEIPTION OF PLATE IV. ^ 



All the shells are considerably enlarged; the apices of each still more 

 magnified. 



Figs. 1, la. Cerithiopsis tuhercularis, Mont. 



2, 2 a. ,, fulchella, Jeffr. 



3, 3 a. „ costulata, MoUer. 



4, 4 a. „ Barleei, JeiFr. 



5, 5 a. „ fayalensis, Wats., n. sp. 



6, 6 a. ,, ?;iara, Wats., n. sp. 



8, 8 «. ,, diadema, Wats., n. sp. 



9, 9 a. „ atalaya, Wats., n. sp. 

 10, 10 a. „ Metaxce, Chiaje. 



The above figures correspond with the numbers in the foregoing description, 

 but No. 7, C. Clarkii, is not here figured, having already been illustrated in 

 Forbes and Hanley, I. c. 



On the Anatomy of the Ambulacra of the Eecent Diadematidce. 

 By Prof. P. Maktin Duncan, V.P. Linn. Soc.,.F.E.S., &c. 



[Read 5th March, 1885.] 

 (Plate V.) 



Contents. 

 I. Introduction. 

 II. The ambulacra of Diadema setosum. 



III. The structure of the edges of the plates (interambulacral and am- 



bulacral). 



IV. The ambulacra of Echinothrix Desori and jS. calamaris. 

 V. The ambulacra of Astropyga radiata and A. pulvinata. 



VI. The ambulacra of Centrostephanus. 

 VII. The ambulacra of Micropyga tuberculata. 

 VIII. The ambulacra of Aspidodiadema microtuberculatum, Agass. 



I. Introduction. 



In a communication on some hitherto unobserved structures 

 of the Arbaciadse, which was read before this Society by Mr. Percy 

 Sladen and myself on February 5, 1885, we stated that the classi- 

 ficatory part of our essay would be given subsequently. 



At that time we were not aware of the bearing of some of the 

 structures of the test of the Diadematidae on the general question 



