HEXACEINID^. 749 



Arthraeantha ithacensis Williams. 

 Plate LXXVL Figs, la, b, c. 



1883. Williams; Proceed. Ainer. Philos. Soc., p. 85, witli a plate. 

 1885. W. and Sp. ; Revision Palffioor., Part III., p. 119. 



Specimens rather below medium size. Calyx obconical, more raiDiiIly 

 spreading to the basi-radial suture than from there upwards. The dorsal 

 cup as wide as high ; the ventral disk flat, a little depressed in the middle. 



Basals forming a low, obconical cwp, with six well defined salient and 

 three re-entering angles; the latter facing the distal ends of the interbasal 

 sutures. Suture lines faintly grooved ; the column facet small and round. 

 Radials rapidly spreading; their upper faces one third wider than the lower, 

 and about equal to the length of the plates; facets somewhat projecting and 

 occupying from one third to one half the width of the upper faces; the limbs 

 at both sides slightly inflected. Costals two, fidly twice as wide as long ; 

 the upper one sharply angular above, its sloping upper faces concave. Arms 

 free beyond the first distichals ; branching, divergent, bifurcating at least 

 twice ; composed of two series of deeply interlocking plates, from which 

 at both sides are given off delicate thread-like pinnules. The anal plate lias 

 the same form as the antero-lateral radials, all being slightly angular below. 

 The arrangement of the plates in the ventral disk is not satisfactorily shown 

 in the specimens ; enough is seen, however, to show that there are five sets 

 of rigid covering pieces above the food grooves, which branch close to the 

 arm bases, each set composed of two rows of plates alternately arranged. 

 The covering pieces are formed into rounded ridges, which grow more prom- 

 inent as they approach the arms. The interbrachials consist of three plates, 

 followed by several rows of interambnlacral pieces, and these by the orals. 

 The anus is excentric, surrounded by a number of moderately small, slightly 

 convex pieces, which form a little rounded protuberance near the outer 

 margin of the disk. Cal3'x and arm plates profusely covered with spine- 

 bearing tubercles, of which there are thirty to thirty-five upon each radial, 

 and a proportionate number upon the basals ; tlie costals apparently have 

 two, the free arm plates and the covering pieces one each. The tubercles 

 are wanting, so far as observed, on the interambnlacral plates, except upon 

 the anal side. The tubercles are of nearly uniform size ; circular, rounded 

 and narrower at the top, and pitted at the apex for the reception of the 



