2/6 



NORTH AMERICAN INDEX FOSSILS, 



separated from the rest of the cephalon by a deep furrow. Tho- 

 racic segments 13. 



Middle Cambric of Utah. 



51. P. housensis Walcott. (Fig. 1574.) Cambric. 

 Only posterior glabellar furrows apparent; fixed cheeks broad. 



Short occipital spine present. Surface of cephalon finely granulosa. 

 Middle Cambric of Utah. 



52. P. piochensis Walcott. (Fig. 1575.) Cambric. 



Fig. 1575. Ptychoparia piochensis, a large head, a small entire individual, and 

 an hypostoma. (After Walcott.) 



Neck ring with a small median node; glabella small, distinctly 

 grooved, anterior cephalic limb broad; thorax of 19 segments. 

 Pygidium small, of three or four united segments. 



Middle Cambric of Nevada. 



53. P. robbi Hartt. (Fig. 1576, c.) 



Cambric. 



Fig. 1576. a, h, Plychoparia adamd, cranidium and entire individual (restored), 

 enlarged ; c, P. robbi, cranidium, enlarged ; d, e, P. ouongondiana, two cranidia of 

 different ages, enlarged ; /, Stremiella strenua, cranidium. (All after Walcott. ) 



Anterior portion of cephalon nearly straight; its width about 

 equal to the entire length of the cephalon. Glabella very convex, 



