300 Field Columbian Museum — Geology, Vol. II. 



the radial plates. The diameter of the dorsal cup at the arm bases 

 is about one-half greater than the height of the cup. The dorsal cup 

 is ornamented with series of prominent rounded ridges. The ridges 

 begin near the middle of the lower margins of the basal plates at the 

 juncture with the column and extend to the center of the plate. 

 At this point they bifurcate and pass to the center of the radials 

 where they join the ridges from the basals on either side. From 

 the center of the radials the ridges follow the median line of the radials 

 and costals to the middle of the axillary costal where they again bifur- 

 cate and follow the distichals up to the arm bases. Another, less 

 conspicuous ridge connects the centers of the posterior lateral radials 

 crossing the first anal plate. Surface of all plates, not ornamented with 

 the ridges, smooth and moderately convex. Sutures not situated in 

 grooves. In the natural casts the dorsal cup is broad and flat at the 

 base and curves abruptly upward at about the middle of the basal 

 plates; it expands moderately up to the middle of the radials, where 

 a slight constriction occurs, and above this constriction the expansion 

 is very rapid until at the arm bases the plates are nearly at right angles 

 to the axis of the calyx. Ventral disc not preserved. 



Infrabasals five, small, scarcely extending beyond the columnar 

 facet. Basals five, large, forming with the infrabasals a shallow cup ; 

 the posterior basal is heptagonal and larger than the others which are 

 hexagonal. Radials five, larger than the basals; the two posterior 

 laterals are hexagonal and the rest are heptagonal. First costals 

 hexagonal, less than half the size of the radials. Second costals 

 axillary, pentagonal or hexagonal. In some cases the interdistichal 

 appears to join the axillary costal. Distichals two or more in each 

 series. First interbrachials are hexagonal, intermediate in size between 

 the radials and first costals ; they are followed by three or more rows 

 of two plates each, diminishing in size upward. Interdistichals pres- 

 ent, but in the specimens at hand only the first plate is preserved; 

 this appears to be hexagonal or heptagonal. The anal interradius 

 is wider than the interbrachial areas. The first anal plate is hexagonal, 

 smaller than the radials ; it rests on the distal edge of the posterior 

 basal and separates the posterior lateral radials; it supports on its 

 distal edge a row of plates, diminishing in size, which lead up to the 

 vault ; the space between this row of plates and the posterior lateral 

 rays is filled with smaller plates. 



In its general form and proportions this species is so unlike all other 

 species of this genus that it will be easily recognized. 



The type specimen, P 889 1 , was collected by the writer in the spring 

 of 1906. It consists of a nearly perfect natural cast of the dorsal 



