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 the 

 type are taken from the cut, those of this specimen from the cast. 



Owing to the fact that this specimen is somewhat distorted, the 

 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 14.3 

 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 specimen is 

 about twice the length of the type and somewhat more slender. The 

 basals, radials, first costals and first interbrachials are much higher 

 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. 



