PLATYCRINIDZ. 675 
Type in the Museum of Comparative Zodlogy. 
Remarks. — The peculiar form of the dorsal cup, the elliptic projecting 
column facet, combined with the arm structure, distinguish this species 
readily from all others known to us. 
TRUNCATULUS GROUP. 
Plates without ornamentation ; costals two. 
Platycrinus truncatulus Hatt. 
Plate LX VII. Figs. 11a, b, and Plate LX_XI. Figs. 15a, b. 
1858. Hat; Geol. Rep. Iowa, Vol. L., Part IL, p. 538. 
1881. W. and Sp.; Revision, Part II., p. 75 (Proceed. Acad. Nat. Sci. Phila., p. 249). 
A small species. Dorsal cup basin-shaped, more than once and a half 
as wide as high, broadly truncate at the bottom, the sides slightly convex 
and moderately spreading. Plates heavy, considering the small size of the 
species; surface smooth ; suture lines not grooved and difficult to see. Arms 
unknown. 
Basal cup very shallow, only the upper angles and the extreme upper 
margins of the sides visible m a side view; these curve abruptly upwards by 
a sharp angle; the lower face perfectly flat, except that the column facet 
is surrounded by a small circular ridge; the interbasal suture lines somewhat 
elevated. Radials one fourth wider at the upper end than at the lower, the 
width of the lower face equal to the length of the lateral faces, the upper 
angles distinctly truncated. Facets narrow, semicircular, the excavation 
limited entirely to the outer edges of the plates, the inner edges forming 
a straight line. Costals two, the lower plate shorter than the upper, twice 
as wide as long and quadrangular, the upper one hexagonal, its upper angle 
truncated, and the distichals separated by a wide gap. Distichals two, as 
long as wide, the second axillary indicating another division above. Ventral 
disk shghtly convex, the plates almost flat. Orals rather regularly arranged, 
and small. ‘The ambulacral pieces rising above the general level; very 
narrow, small, and as regularly arranged as in any of the recent Crinoids. 
Interambulacral plates numerous; the first of the regular sides very large, 
its sides resting against the first costals, the two at the sides, which in this 
species are extremely small, touch but slightly the second; the plates above 
very minute. At the anal side there are three plates abreast; the middle 
