164 MK. J. W. GREGOEY OK AE,CH.i;OPjSrEUSTES ABRTJPTUS. 



round ; the outer elongate, especially in the postero-lateral ambu- 

 lacra, in which they become comma-shaped. The petals reach to 

 the ambitus. The interporiferous areas are broad, and taper but 

 slightly at the lower end. 



Interradii: The postero-lateral are the broadest. In all the 

 plates are massive, somewhat Y-shaped, and imbricate slightly. 



Periproct : large and marginal ; it is elliptical, with the longer 

 axis vertical. 



Peristome: less excentric than in some species. Strongly bi- 

 labiate. 



Dimensions of type : — 



mm. 



Length 87 



Height 60 



Width 95 



Distance of apical system from anterior margin. 29 



mouth „ „ „ 27 



Antero-lateral ambulacra : length 72 



„ „ width 16 



Postero-lateral ambulacra : length 77 



„ „ width 19 



Type. Brit. Mus., E 3433. (G. Pirth Pranks Coll.) 



JDistribution. Uppermost limestone of the Oceanic Series. Bissex 

 Hill, Barbados. 



Affinities and Distinctions. Prom A. hystrix (A. Agassiz) this 

 species differs in the more anterior position of its apical disk, but 

 this may be exaggerated by the slight crushing back of the anterior 

 margin of the specimen ; moreover, the plates in the new species 

 are Y-shaped, whereas Prof. Agassiz figures the margins in his 

 species as horizontal and nearly straight. Prom A. cuhensis (Cott.) 

 it may be distinguished by the fact that, owing to the more central 

 position of the apical system in the Cuban species, the antero-lateral 

 ambulacra run obliquely forward and are not in the same straight 

 line. 



Prom Pdlceojpneustes cristatus (A. Ag.) and P. Jimenoi (Cott.) it 

 differs in that the petaloid portions of the ambulacra in these are 

 short, and the pores become single and minute at some distance 

 from the ambitus ; the shape of these three species is also different. 



11. Zoological Apeinities. 



The irregular toothless Pchinoidea are divided into two orders, 

 the Cassiduloidea and Spatangoidea ; the former is characterized by 

 the central or subcentral mouth surrounded by the depressed widened 

 'phyllodes' of the ambulacra, and the raised *bourrelets' between 

 them, which together form the ' floscelle.' The Spatangoidea on 

 the other hand possess no floscelle, have the mouth generally very 

 excentric anteriorly, and the anterior ambulacrum much reduced. In 



