6 



part curves over horizontally, in the direction of the arms. In the 

 ray on the right of the azygous area, in the specimen illustrated, 

 the first tertiary radials are somewhat trapezoidal in outline, and 

 do not cover the superior faces of the first primary radials, and 

 the second tertiary radials rest on both the secondary and first 

 tertiary radials; they are axillary and bear upon each upper slop- 

 ing side a free arm, which gives to this ray four arms. In the ray 

 on the left of the azygous area the second secondary radial, on 

 the proximal side, bears a single arm, there is an intercalated 

 large plate resting in the angle formed by the two first primary 

 radials, which is axillary and bears upon each superior side a ter- 

 tiary radial that supports a single arm, which gives to this ray 

 three arms. Each of the lateral rays is constructed in the same 

 way, and each one bears three arms. In the ray opposite the 

 azygous area each second secondary radial bears a single arm, 

 which gives to this ray two arms. There are, therefore, fifteen 

 arms in this species, and fifteen ambulacral openings to the 

 vault, which are very conspicuous in a summit view. The second- 

 ary and tertiary radials are tumid and pyramidal plates. One of the 

 other specimens has the four-armed ray on the left of the azygous 

 area, and the three-armed ray on the right. We have elsewhere 

 shown that, in our opinion, this variation should not be regarded as 

 of specific importance. The arms are composed of a double series of 

 interlocking plates and each one is in the form of a sharp angular 

 ridge on the lower side. 



The first regular interradials rest between the superior lateral sides 

 of the first primary radials and extend as high as the third primary 

 radials. In the area adjoining the four-armed ray, it is followed by 

 two plates that separate the secondary radials and first arm plates 

 and curve over the margin of the calyx and unite with three plates 

 belonging to the vault. In the area next to the left of the azygous 

 area there are also three plates, but one of them is intercalated and 

 rests on the top of the first secondary radial, and is cut off from unit- 

 ing with the plates of the vault. In each of the other two areas there 

 are only two plates, the second ones of which curve over the margin 

 of the calyx and unite with the plates of the vault. Each of these 

 plates in the second, ranges are longitudinally convex or bear an 

 elongated node. There are four plates in the azygous area. The first 

 one is in line with the first primary radials and of about the same 



