Trii.oi!! lES OK iiiK Chazv Limestonk. 359 



as bearing spines, nor are any s])ines mentioned as occurring on the 

 cranidiuni. Professor Whitfield mentions an associated hypostonia, 

 but his material lacked the free cheeks and the details of the pygidium 

 could not be made out. The specimens were, however, very small, 

 the thorax described being only six twentieths of an inch in length. 

 In discussing the generic relations of this species, Whitfield says : 

 "The next nearest genus might he Acidaspis. In the structure of 

 the pleura this species differs quite remarkably from Sao hirsiifa, being 

 between that and Aa'daspis, having a central ridge which becomes 

 obsolete, however, on the outer ])ortions. Also it differs in the back- 

 ward bending and pointed structure of their extremities." 



Now that specimens have been found which show the si)ines it be- 

 comes evident that this peculiar species is an Acidaspis, and one of 

 the most sj^nose trilobites in the American Ordovician. 



Description. 



Cephalon broad, roughly semicircular, spinose, cranidium large, 

 extending nearlv the whole width of the cephalon and terminating 

 anteriorly in a sort of ring-shaped ])rojection on which are five long 

 slim spines which point forward and upward at an angle of about 

 forty-five degrees. Fixed cheeks prominent, convex, sloping rapidly 

 to the sides. Glabella very convex, separated from the fixed cheeks 

 by a deep furrow which extends around the front and makes a little 

 more than half a circle. The anterior pair of glabellar furrows are 

 short and directed a little backward, making only a sort of a dent in 

 the side of the glabella near the front. Back of these furrows there 

 is, on each side, a pit parallel to the axis of the glabella, making two 

 small convex side lobes. Over the fixed cheeks and glabella are scat- 

 tered spines of different sizes. On the top of the glabella and close 

 to the axis are two rows of large spines, three in each row. On each 

 fixed cheek is a row of large spines parallel to the furrow outlining 

 the glabella. There are three or four spines on each side. Inter- 

 spersed with these are a large number of smaller spines, all pointing 

 upward and forward. 



The neck furrow extends all across the cranidium, and the neck seg- 

 ment is narrow and convex. At each lateral angle is a long thin 

 spine which extends out horizontally and turns a little backward. 



Free cheeks large, with a narrow, convex, border which bears num- 

 erous small spines. At the genal angle the cheek bears a short, 



