PLATYCEINID^. 6T5 



Typz in the Museum of Comparative Zoology. 



Eemarks. — 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. 



TBUNGATULUS GROUP. 

 Plates without ornamentation ; costals two. 



Platycrinus truncatulus Hall. 

 Plate LXVII. Figs. 11a, h, and Plate LXXI. Figs. 18a, h. 



1858. H4LI, ; Geol. Eep. Iowa, Vol. I., Part II., p. 538. 



1881. W. and Sp. ; Revision, Part II., p. 75 (Proceed. Acad. Nat. Sci. Pliila., 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 shghtly convex 

 and moderately spreading. Plates heavy, considering the small size of the 

 species ; surface smooth ; suture lines not grooved and difficult to see. Arms 

 imknown. 



Basal cup very shallow, only the upper angles and the extreme upper 

 margins of the sides visible in 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 slightly convex, the pla.tes 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 Cnnoids. 

 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 



