54 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



Abactinal area of rays occupied by tkree columns of large, thick, 

 more or less tumid or carinate, closely adjoining, tuberculate plates; 

 more or less of the iiiframai^inal plates are also to be seen, making 

 five visible columns of plates on each ray abactinally. There is, 

 therefore, a single radial column bounded on each side by two columns 

 of supramarginal and two of inframarginal plates. The latter 

 columns are the marginal plates both actinally and abactinally, but 

 on the dorsal side are overlapped by the supramarginals. In the 

 center of the disk there is a small but very tumid plate (central 

 disk plate) around which is a nearly complete circle of smaller acces- 

 sory plates and an open space which may represent an anal opening. 

 Outside of this first circle of accessory plates there is a ring of 10 

 very large plates that in the geologically later species are stellate 

 on the surface, 5 of which are radial and 5 interradial in position. 

 The radial plates are the basal radial plates of the 5 medial columns, 

 while the 5 interradial plates give rise to the 10 larger and conspicu- 

 ous supramarginal ossicles. The basal supramarginals adjoin one 

 another in the axillary areas, and in the geologically later forms 

 these also have stellate surfaces. Between each of the 5 inter- 

 radial plates there is inserted a single plate, making a second but 

 disconnected ring of accessory plates. 



Madreporite abactinal, more or less large, adjoining one of the 

 large interradial plates and between the basal plates of two supra- 

 marginal columns of ossicles of adjoining rays. 



No accessory ambital or abactinal plates. 



Spines appear to be absent everywhere except on the inframarginal 

 and adambulacral ossicles. Otherwise the surface is granular or 

 tuberculate. 



Inframarginal plates well-developed, tuberculate, and common 

 to both the actinal and abactinal areas. These proximally adjoin the 

 five isolated and large axillary marginals, each one of which occupies 

 an axil of the rays. Each plate, except the axillary ones, bears on 

 its outer edges one to three apparently smooth, small spines. 



Axillary plates quite conspicuous, marginal, and one in each 

 actinal axil. In at least one of the species there is a single tiny acces- 

 sory piece orad to each axillary {H. narrawayi), and it is only seen 

 on the inside of the skeleton. 



Adambulacral plates numerous, one or two to each inframarginal 

 plate and two on each side of each axillary marginal plate. All of 

 these pieces bear at least two spines, one on the iimer and another 

 on the outer edges. 



Oral armature consisting of at least five pairs of pointed and elon- 

 gate basal adambulacral plates; each bears spines like the other 

 ambulacrals. In rare cases issuing from beneath these ossicles may 

 be seen five single stout spines or plates (tori) that are connected 



