TRILOBITA—OPISTHOPARIA. 



275 



XXI. Ptychoparia Corda. 

 Cephalon with a narrow, raised marginal rim. Glabella narrow- 

 ing anteriorly; furrows present. Segments of thorax usually 13 

 to 15. Pleura with backward pointing extremities. Pygidium 

 moderately large. Cambric-Ordovicic. 



Fig. 1572. a, b, Zacanthoides idahoensis ; b, free cheek ; c, Albertella helena. 

 ( A.fter Walcott. ) 



49. P. adamsi Billings. (Fig. 1576, a, &.) Cambric. 

 Ocular ridge well developed. Form as in figure ; glabella nearly 



parallel-sided. 



Lower Cambric (Georgian) of Vermont and Quebec. 



50. P. kingi Meek. (Fig. 1573.) Cambric. 

 Glabella short depressed until nearly on a level with the cheeks. 



Fig. 1 573' Ptychoparia kingi, the type 

 specimen. (After Walcott. ) 



Fig. 1574. Ptychoparia housen- 

 sis, the type specimen, enlarged. 

 (After Walcott.) 



k 



