PLATYCEINID^. 697 



Basal cup somewhat obconical, with a slight truncation at the lower end, 

 surrounded by a small circular ridge ; height equal to one half the leno-th of 

 the radialsj interbasal sutures obsolete. Eadials longer than wide in the 

 adult, a little wider in young specimens ; the lower face rounded at the 

 margin ; the sides shghtly curving inward near the top 5 the upper face 

 almost straight, sloping very little at the outer ends ; the outer face ab- 

 ruptly elevated beneath the facet, forming a prominence or node about 

 equal in size to the costal. Facets very small, directed upwards, occupying 

 rarely more than one fourth the width of the radials, and not entering 

 the body of the plate, or only very slightly. Costals irregularly pentangu- 

 lar, the sloping upper faces concave, the angle sharp. Distichals two, as 

 long as wide, giving off two arms from each side, which remain simple. 

 Arms rather small, composed of moderately long, cuneate pieces, which 

 gradually interlock, and from the radials up are constricted at the middle ; 

 the sockets for the attachment of the pinnules projecting. Pinnules not in 

 contact, rather large. Interradial plates erect, five so far as observed ; the 

 middle one large, subquadrangular — that of the anal side widest — it is 

 flanked at each side by two small pieces, which together rarely reach the 

 height of the middle one, and both are in sutural contact with the distichals, 

 thus being strictly interbrachial in position. All other parts of the tegmen 

 imknown. 



Horizon and LocaVdy. — Lower Burlington limestone, Burlington, Iowa. 



Type in the Museum of Comparative Zoology. 



Remarks. — This species resembles P. scdhina M. and W. ; but is readily 

 distinguished by its much smaller and sharper nodes, which are sparingly 

 scattered upon the surface ; while those of that species are densely crowded. 

 It is also remarkable for the abrupt elevation beneath the radial facet, which 

 appears almost as if constituting a part of the costals. 



Platycrinus geometricus W. and Sp. (nov. spec). 



PlaU LXVII. Fig. 10, and Plate LXVIII. Fig. 8. 



A small species somewhat resembling P. scoUna, from which it differs in 

 the greater height of the dorsal cup, in the ornamentation, and in the pecu- 

 liar form of the arm plates. Dorsal cup bell-shaped, about as high as wide, 

 slightly spreading upwards. Surface of plates covered with knife-like, con- 

 spicuous, undulated ridges or rows of sharp, confluent nodes, passing out 



