38 riilMOKDlAL TEKIOD. 



nbout one-quarter that of the whole width of the shell. This, of course, 

 indicates the existence of a shallow, tapering, longitudinal groove upon the 

 iinier surface of the shell ; but, so far as observed, there is no external indi- 

 cation of its presence. Surface marked by lines and undulations of growth, 

 which are apparently strongest upon the flattened side. 



Length, from fifteen to eighteen millimeters. 



In the case of simple forms like these, it often seems necessary to take 

 into careful consideration diiferences that, in more complex forms, would be 

 considered as merely individual, or slightly varietal modifications, and there- 

 fore disregarded. The diflFerence between our specimens and those of 

 Professor Hall, as represented by his figures and description, seems to be of 

 this slight character ; and it is only in view of the fact just stated that doubt 

 is felt as to the specific identity of our shell with H. primorclialis. The 

 principal external differences (the internal characters of Professor Hall's 

 shell have not been made known) are that our forms are a little more 

 robust, a little less flattened upon the dorsal side, and the convexity of the 

 lines of growth and of the margin of the aperture at that side are a little 

 greater. From H. gregaria Meek and Hayden, it differs in its greater size, 

 its concentrically-marked surface, and in the outline of its apertm-e. 



Position and locality. — Strata of the Primordial period, and probably of 

 the Potsdam epocli, Pioche, Nevada. 



ARTICULATA. 



Class CRUSTACEA. ' 



Order TRILOBITA. 

 Family AGNOSTID^. 



Genus AGNOSTDS Brongniart, 1821. 



Agnostus interstrictus White. 



Plate II, fig. 5 a and ft. 



Agnostus interstrictus White, 1874, Geog. & Geol. Exp. & Surv. west 100th Merid., 

 Prelim. Rep. luvert. Foss., 7. 



Head and pygidium of almost exactly equal size and general shape 

 and otherwise closely resembling each other. 



