NO. 3 CAMBRIAN AND OZARKIAN TRILOBITES 97 



Genus IDDINGSIA Walcott 



Iddingsia Walcott, 1924, Smithsonian Misc. Coll., Vol. 75, No. 2, p. 58. 



Description. — Iddingsia is based on the cranidium and includes 

 a small group of species present in the Great Basin and eastward. 



The cranidium of Iddingsia is distinguished by its nearly flat, 

 sloping, equally divided frontal border, strongly developed glabella 

 and narrow fixed cheeks. It differs from Diinderbcrgia and Modocia, 

 with which it is associated, in the equal width of its border and rim. 

 The ocular ridges are also quite prominent. 



The other parts are unknown. 



Genotype. — Ptychoparia similis Walcott. 



Range. — Upper Cambrian, Eureka District, Nevada. 



The generic name is in memory of Dr. Joseph P. Iddings, geologist, 

 who was my associate in the survey of the Eureka District in 1880. 



IDDINGSIA ROBUSTA (Walcott) 



Plate 16, figs. io,» II 



Ptychoparia similis robnstus Walcott, 1884, Monogr. U. S. Geol. Surv., Vol. 8, 

 p. 53, pi. I, figs. 9, 9a. (Original description and illustrations.) 



This species varies slightly from /. similis in the relative propor- 

 tions of the parts of the cranidium, sufficiently to warrant regarding 

 it as a full species rather than a variety. 



Formation and locality.— Same as for /. similis. 



IDDINGSIA SIMILIS (Walcott) 

 Plate 16, figs. 8, 9 



Ptychoparia similis Walcott, 1884, Monogr. U. S. Geol. Surv., Vol. 8, p. 52, 

 pi. ID, fig. 10. (Original description and illustration.) 



Iddingsia siuiilis Walcott, 1924, Smithsonian Misc. Coll., Vol. 75, No. 2, p. 58, 

 pi. 12, fig. 6. 



The original description and figures 8 and 9 give all that is known 

 of this species. 



Formation and locality. — Upper Cambrian : (60) Secret Canyon 

 shale, near Richmond Mine. (61) Near Hamburg Mine, Eureka 

 District, Nevada. 



Genus IRVINGELLA Ulrich and Resser 



Irvingella Ulrich and Resser in Walcott, 1924, Smithsonian Misc. Coll., 

 Vol. 75, No. 2, p. 58. 



Description. — Glabella large, the dorsal and occipital furrows very 

 strong and usually rather wide ; the posterior pair of glabellar fur- 



