302 FIELD CoLUMBIAN MusEuM— GEoLoGy, VoL. II. 
resting upon the short, sloping edges of the first distichals and be- 
tween the second distichals. 
Arms and ventral disc not preserved. 
The following dimensions are given for comparison. Those of tke 
type are taken from the cut, those of this specimen from the cast. 
i This specimen Type specimen 
Length of dorsal cup, ey ies aa 19.0 
Diameter at arm bases, 18.2 C355 
Diameter at top of radials, II: I 8.0 
Height of radials, eet 9 
Height of basals, as 235 
Owing to the fact that this specimen is somewhat distorted, th . 
dimensions of the diameters as given are estimated. The actual 
measurements are as follows: 
The longer and shorter diameters at arm bases are 22.15 and oe 3 
respectively. 
The longer and shorter diameters at top of radials are 12.9 and 9.3 
respectively. 
The specimen here described ( Mus. No. P 8879 ) consists of a 
practically complete natural cast accompanied by a portion of the mold. 
The mold comprises one complete ray, a portion of two others and part 
of three interbrachial areas. So far as known to the writer, this is 
the first specimen of this species in which the external characters 
of the plates have been described or figured. This specimen differs 
from the type as figured by Hall in several particulars, and if other 
specimens should be found showing these characters to be constant, it 
may be advisable to consider this a new species. This specimea is 
about twice the length of the type and somewhat more slender. The 
basals, radials, first costals and first interbrachials are much higrer 
in proportion to-their width. In the type the sutures between the first 
costals and first interbrachials are straight, but in this specimen they 
are arched, with the convex side towards the interbrachial. 
This species is distinguished from all other species of the genus 
by the absence of a basal cavity. 
Locality: Niagaran limestone of the spoil heaps along the Chicago 
Drainage Canal near Lemont, Illinois. 
