RHODOCRINID^. 257 



The lower distichals to the number of seven or eight take part in the calyx; 

 thej decrease rapidly in size, the lower one being comparatively large, but 

 the fourth is but little larger than the free plates ; the plates above are tri- 

 angular and interlock. Arms long, rather stout at their bases, but tapering 

 gradually to the tips where they are quite thin ; they bifurcate twice or 

 oftener, and are composed of two rows of plates with parallel faces. Regu- 

 lar interbrachials : 1, 2, 3, 3, followed by numerous smaller plates ; the 

 anal side has 1, 3, 3, 4, etc. Of interdistichals as many as fourteen pieces 

 have been observed, the lower one resting upon the first distichals. Con- 

 struction of the ventral disk and form and position of the anal opening 

 unknown. Column round, tapering downward; the upper part composed 

 of alternately large and small joints with angular edges ; but at about three 

 inches from the calyx the internodal joints reach the width and height of 

 the nodal ones, and the edges lose their angularity. 



Horizon and Locality, — Trenton limestone ; Ottawa, Canada. 



The figured specimens are in the collection of Mr. John Stewart of 

 Ottawa. Ttjpe in the Geological Survey Museum at Ottawa. 



ArchSDOCrinus desideratUS W. E. Billings. 

 Plate X. Figs, Jfa, h, 



1885. W. R. Billings; Trans. Field Natur. Club, Ottawa No. 6. 

 1885. W. and Sp. ; Revision Palseocr., Part III., p. 98. 



A rather large species. Calyx depressed globose ; basal portions deeply 

 depressed, forming a broad funnel-shaped pit, much wider than the circum- 

 ference of the stem, and deep enough to contain five or six stem joints. 

 Plates convex, without ornamentation or other markings, except obscure 

 elevations — a kind of broad, rounded ridges — following the median line 

 of the rays, and branching to the basals. 



Infrabasals small, hidden by the column. Basals large, twice as long as 

 wide; the lower half of the plates forming the sides of the basal pit; the 

 upper halves, which bend abruptly upward and outward, occupy the sides 

 of the cup. Eadials pentagonal, almost as long as wide, their lower faces 

 forming a sharp angle, which extends down to the margin of the basal 

 concavity. First costals hexagonal, as wide as the radials, but shorter; the 

 second shorter and narrower than the first. Distichals 4X2, all incorporated 

 into the calyx, and comparatively small ; they are rounded exteriorly, and 



33 



