110 PEOE. P. M. DUNCAN OlS" THE ANATOMY OY THE 



Plate 2 is a primary, comprising the matnelon and much of the 

 base of the boss of the tubercle, and a large part of the surface 

 at the median suture. 



The plate 3 is a well developed primary which pushes plate 2 

 abactinally, and forms a part of the boss of the tubercle and also 

 of a small part of the compound plate near the median line. 



AstroTpyga pulvinata, Agass. — The details of the ambulacra of 

 this species resemble those of Astropyga radiata, allowance 

 being made for the difference in the dimensions of the tubercles 

 in the two species. 



The lapping of the coronal plates of the interradia is well 

 seen in these forms ; and the underlap of the aboral edge of the 

 interradial plates at the vertical suture, by a thin process of the 

 adoral edge of the abactinally placed plate, is distinct. 



I have failed to notice the division of the interradial plate 

 above the ambitus, described by A. Agassiz, in both species. 



It will have been noticed that whilst the triplets of Astropyga 

 are arranged, not after the type of the Triplechinidse, but after 

 that of Diadema, they differ from those of the last-named genus 

 by being here and there made into demi plates, not, however, 

 after the type of Strongylocentrotus, but rather after that of 

 Codofleurus. 



The union of the two sets of triplets in one great plate is 

 very remarkable. 



VI. G-enus Centeostephanfs, Peters, 1855. 



A. Agassiz appears to consider this genus in the light of a 

 subgenus of Diadema, for the word Diadema is placed in brackets 

 before the generic term in the ' Eevision.' Certainly the external 

 resemblances of Diadema and the genus are great, and the 

 presence of ten buccal plates with spines and pedicellariae, the 

 principal peculiarity of GentrostepJianus, is only subgeneric. 



The only specimen that I have seen shows that the ambulacra 

 have the Diadematid character. 



VII. G-enus Miceoptga, A. Agassiz, 1879. 



Micropyga tuberculata, A. Agassiz, is fully described in the 

 Eeport on the ' Challenger ' Echinoidea, p. 68, pi. vii. 



