INVERTEBRATES. 193 



stria?, and provided with a small, scarcely projecting rim, 

 which is not interrupted by the sutures — perforated by a nearly 

 round, central opening, near one third the diameter of the 

 head of the column. First radial plates large, the posterior 

 ones being smaller, and proportionally narrower than the 

 others; all generally longer than wide; three of them hexa- 

 gonal, and two heptagonal. Second radials, rather smaller than 

 the first, about as wide as long, all hexagonal. Third radials, 

 rather smaller than the second, generally a little wider than 

 long, heptagonal in form, and each supporting on its superior 

 sloping sides, two smaller heptagonal or octagonal secondary 

 radials, which appear to have been each surmounted by two 

 tertiary radials, or possibly in part, brachials. Resting between 

 the superior sloping sides of the third radials, is a small hexa- 

 gonal, interaxillary piece, evidently surmounted by others, the 

 number and forms of which are unknown. 



First anal plate nearly as long as the first radial on each 

 side of it, but narrower and distinctly smaller than the ante- 

 rior first radial, heptagonal in form, and surmounted by two 

 much smaller hexagonal, and one pentagonal pieces in the 

 second range; in the third range there are four still smaller 

 pieces, and above these others, the number, form and arrange- 

 ment of which cannot be determined in the specimens described. 

 First interradial plates each nearly or quite as large as the 

 second radials, hexagonal or heptagonal in form, and sur- 

 mounted by two or three smaller pieces; above the latter there 

 are apparently about three other ranges of two pieces each, 

 which is as far as our specimen shows them. 



The surface of the plates is neatly ornamented by narrow, 

 sharply elevated ribs, about four to six of which radiate from 

 the central region of each plate, to each of its sides, excepting 

 below the middle of the first radials, where there are usually 

 about eight or ten. The costse on the other plates are less 

 numerous in proportion to the size of each, and like those on 



25 Sept. 1,1866. 



