334 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



orals. Their shape is somewhat quadrangular, two of their angles pointing ver- 

 tically upward and downward, the other two laterally toward each other. Their 

 lower angles are received between the upper angles of the basals, wliile on their 

 upper, which are somewhat truncated, the narrow first primibrachs are super- 

 imposed. Considerable spaces still exist between the adjacent radials, 'except 

 where the radianal is intercalated in the series, and these are filled only by sarcodic 

 substance. The central portion of these radials is thickened by the endogeneous 

 extension of the calcareous reticulation, and this extends toward its upper angle 

 so as to form a kind of articular surface for the support of the first primibrachs, 

 but it does not extend over the lateral or alar expansions of these plates, which still 

 retain their original condition of cribriform films. The first primibrachs differ 

 considerably from the radials in shape, being rather rods than plates, but they are 

 deeply grooved on their oral aspect, that part which is subsequently to become a 

 central canal being not yet closed in. The calcareous reticulation of their outer 

 or aboral surface is cribriform, but the ingrowth from which they derive their 

 solidity is produced by the development of fasciculated tissue analogous to that of 

 which the columnals are composed. The same general description applies to the 

 second (axillary) primibrachs, wliich, like the first, are nearly cylindrical at their 

 proximal extremities, but expand toward their distal ends so that each presents 

 two articular surfaces on wliich are superimposed the pair of first brachials. The 

 orals, which alternate with the first primibrachs, though somewhat internal to 

 them, now present somewhat of a triangular form, their apices pointing upward; 

 their basal angles, however, are blunted by the encroachment of the radials. At 

 no part of their contour have these plates any definite margin like that which 

 borders the two lower sides of the basals, but the calcareous reticulation of wliich 

 they are composed is continued into the layer of condensed sarcode with wliich 

 they are invested. Although the form of these plates is generally triangular, their 

 surface is neither a plane nor a spherical triangle, but presents a remarkable uneven- 

 ness. Near the apex of each there is a deep depression externally and a corre- 

 sponding projection internally, and the effect of this projection seems to be that 

 when the apices of these plates incline to one another so as to form a five-sided 

 pyramidal cover to the calyx, the plates will close together, not merely at their 

 apices and lateral margins, but also at the upper part of their internal surfaces. 

 There is also a broad depression near the base of each plate, so that its lower margin 

 is somewhat everted. The anal, wliich is intercalated between two of the radials, 

 has a tolerably regular circular shape, but it consists only of a single cribriform film 

 and has no definite border. 



W. B. Carpenter states that the radianal " anal" is still distinguishable in speci- 

 mens of Antedon lifida that show no vestiges of the orals, but it has undergone no in- 

 crease in superficial dimensions and is so far from being augmented in thickness that 

 it seems rather to have been thinned by incipient resorption over its whole surface 

 preparatory to its complete disappearance a short time after. Carpenter did not 

 find that either the upper part of this plate disappears before the lower, or the lower 

 before the upper; and as he found no vestiges of it, though he carefully searched 



