TRILOBITA— PROP ARIA. 325 



Odoiitocephalus Conrad includes those with only the frontal 

 border of the cephalon with a series of spines which are in contact 

 at their outer edges. 



Frontal margin of cephalon : 



a. Slightly crenulate 184. D. pletiroptyx. 



b. With a row of short spines (also on lateral margins) 181. D. dentatus. 



c. With a series of spines in contact at their outer edges 187. D. selenurus. 



d. With a single long spine *. 



'^. Pygidium ending in a long spine 182. D.nasutvs. 



'^. Pygidium ending acutely, without a spine 180. D. vigilans. 



e. Acute 178. D. lirnulurus. 



f. Rounded **. 



**. Neck ring with spine 185. D. anchiops. 



**. Neck ring without spine |. 



f. Pygidium with a row of spines upon the axis 186. D. calypso. 



f. Pygidium without spines upon axis %, 



%. Pygidium narrow, elongate-triangular 177. D. achates. 



\. Pygidium broadly triangular \. 



\. Axis with 20 to 21 annulations 183. D. mururus. 



\. Axis with 12 to 13 annulations 179. D. dance. 



177. D. achates Billings. (Fig. 1640.) Ordovicic. 

 Frontal margin of cephalon broadly curved. Pygidium narrow, 



elongate, triangular, with about 14 annulations in the axis and 

 about 10 pleura. 



Trenton of New York, Ohio and Ontario. 



178. D. limulurus Green. (Fig. 1641.) Siluric. 

 Cephalon pointed anteriorly. Glabellar lobes broad anteriorly 



and narrow posteriorly. Pygidium triangular and ending in a 

 long slender spine; axial annulations 15; pleura grooved medially. 

 Entire surface granular. 



Niagaran of New York, Ohio, Tennessee and Ontario. 



179. D. danae Meek and Worthen. Siluric. 

 Dififers from D. limulurus in its larger size, in the absence of the 



marginal rim anterior to the glabella, in the more rounded outline 

 of the pygidium, with fewer (12-13) annulations of the axis. 

 From D. pleuroptyx and D. micrurus, it differs in the lesser number 

 of segments of the pygidium and in the less granular surface. 

 Niagaran of Illinois. 



180. D. (Synphoria) vigilans Hall. Siluric. 

 Cephalon with pointed genal spines and a long slender upward- 

 curving spine at the frontal margin. Frontal lobe occupying about 



