TRILOBITA—OPISTHOPARIA. 273 



pleura of the pygidium present in 0. wasatchensis and 0. 

 nevadensis. 



Lower? Cambric of Vermont and Quebec. 



42. 0. wasatchensis Hall and Whitfield. (Fig. 1568.) Cambric. 

 Ocular ridge near margin of cephalon. Pygidium with narrow 



pleural grooves, and the three posterior marginal spines shorter 

 than the rest. 



Middle Cambric of Utah and Nevada. 



43. 0. curticei Walcott. (Fig. 1569, c.o?.) Cambric. 

 Form as in figure; marginal spines of pygidium rounded and 



long. 



Middle Cambric of Georgia. 



44. 0. nevadensis Meek. Cambric. 

 Larger than preceding; length of thorax 1.7 inches, breadth 2.5 



inches; pleural grooves broader. (Type of genus.) 

 Middle Cambric of Utah and British Columbia. 



XIX. Zacanthoides Walcott. 

 Differs from Olenoides in the spine on the posterior end of the 

 fixed cheek, in the larger eyes, situated nearer to the glabella, in the 

 more oblique pleural grooves and longer pleural spines, and in the 

 narrow pygidium which is composed mainly of the axis with two 

 or three spinose segments turned abruptly backward. Thorax of 

 9 segments; a long spine on the axis of the next to the last seg- 

 ment. Cambric. 



45. Z. typicalis Walcott. (Fig. 1570.) Cambric. 

 Glabella subquadrangular ; pleura very short, with long spines. 



Pygidial spines narrow. (Type of genus.) 

 Middle Cambric of Nevada. 



46. Z. spinosus Walcott. (Fig. 1571.) Cambric. 

 Glabella broader than in Z. typicalis and spinose segments of 



pygidium broader. 



Middle Cambric of Nevada and British Columbia. 



47. Z. idahoensis Walcott. (Fig. 1572, a, &.) Cambric. 

 Pleural spines longer and directed more strongly backward than 



in Z. spinosus. Glabella subquadrangular. 

 Middle Cambric of Idaho. 



