INVERTEBRATES. 203 



erate size, more than twice as wide as high, truncated and 

 concave below, where it is as wide as at the summit, and pro- 

 jects below the end of the column; margins so deeply notched 

 at the sutures as to present a distinctly trilobate appearance as 

 seen from below; columnar facet round, about one-third as 

 wide as the base, and provided with a minute central perfora- 

 tion. First radial pieces a little wider than long, two of them 

 heptagonal and three hexagonal, widening from the base to 

 the lateral angles, and each provided with a prominent, com- 

 pressed, central node, extending somewhat obliquely outwards 

 and downwards, from which one or two rather angular ridges 

 pass down upon the basal plates. Second radial pieces between 

 one-half and two-thirds as large as the first, distinctly convex, 

 or each having a more or less distinct central node, about one- 

 third wider than high; two of them pentagonal and three hex- 

 agonal. Third radials as wide as, or wider than, the second, 

 but rather shorter, three pentagonal and two heptagonal, sup- 

 porting on each of their superior sloping sides a smaller second- 

 ary radial piece, each of which is surmounted by two smaller 

 pieces, which, if they are both brachials, would make four 

 arms to each ray, or twenty in all; but there are some appear- 

 ances of another bifurcation on each inner piece, which would 

 make thirty arms. 



The first interradial pieces are about as large as the second 

 radials, and, like them, are each provided with a central node. 

 They are as long as wide, regularly hexagonal, and support on 

 their superior sloping sides two smaller pieces in the second 

 range, and two or three in the third, which connect with the 

 vault above and with the brachial pieces on each side. The 

 first anal plate is sometimes nearly as large as the first radials, 

 hexagonal in form, and supports in the second range, two 

 rather smaller hexagonal or heptagonal pieces; above these 

 there are two others, which connect with the third radials and 

 secondary radials on each side, and with the vault above. 



