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PLATYCRINIDE. 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 length of \ 
the radials; interbasal sutures obsolete. Radials longer than wide in the | | 
adult, a little wider in young specimens; the lower face rounded at the | 
inargin; the sides slightly curving inward near the top; the upper face | 
almost straight, sloping very little at the outer ends; the outer face ab- | 
seg ta abate na IN ee Ae 
— ease PS rg tu Lh VAN 7 bi 
— 
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 | 
unknown. 
Florizon and Locality. — Lower Burlington limestone, Burlington, Iowa. 
Type in the Museum of Comparative Zodlogy. 
Remarks. — This species resembles P. scobina 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. 5 
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.). 
Plate LXVIL. Fig. 10, and Plate LXVIII. Fig. 8. 
A small species somewhat resembling P. scodina, 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 
88 | 
