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HEXACRINID ZA. 749 
Arthracantha ithacensis Wrt1rams. 
Plate DXXVI. Figs. La, 6, ec. 
1883. Witurams; Proceed. Amer. Philos. Soc., p. 85, with a plate. 
1885. W. and Sp.; Revision Paleocr., Part III., p. 119. 
Specimens rather below medium size. Calyx obconical, more rapidly 
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 cup, 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 hmbs 
at both sides slightly inflected. Costals two, fully 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 has 
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 interambulacral 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. Calyx 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; the costals apparently have 
two, the free arm plates and the covering pieces one each. The tubercles 
are wanting, so far as observed, on the interambulacral 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 
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