2 56 NORTH AMERICAN INDEX FOSSILS. 



88. Pygidiutn triangular LXIX. Dalmanites 



88. Pygidium semicircular gg 



gg. Margin entire LXVIII. Pterygometopus 



gg. Margin spinous 7+ 



7t. Eyes large LXX. Cryphceus, 



7t. Eyes small LXIII. Ceraur 



Order HYPOPARIA Beecher. 



I. Agnostus Brongniart. 



Small. Cephalon and pygidium subequal in form, size and 

 markings. Free cheeks ventral. Eyes absent. Glabella not extend- 

 ing to anterior border of cephalon. Thorax of two segments, 

 with grooved pleufa. Cambric-Ordovicic. 



I. A. interstrictus White. Cambric. 



Fig. 1543, a^ b, Agnostus acadicus, cephalon and pygidium (enlarged) ; c-e, 

 Microdiscus speciosus, entire individual and pygidium enlarged ; /, g^ M, lobaius, 

 cephalon and pygidium much enlarged ; h, i, M. pulchellus, cephalon and pygidium. 

 (After Walcott.) 



Thorax narrower than cephalon or pygidium. Differs from 

 A. pisiformis in the almost total absence of basal lobes in the 

 glabella, in the presence of a slight folding back of the marginal 

 rim at the antero-lateral angles of the pygidium, and in a less 

 definite segmentation of the axis of the pygidium. 



Middle Cambric of Utah and Nevada. 



2. A. acadicus Hartt. (Fig. 1543, a, &.) Cambric. 



Cephalon and pygidium very depressed-convex. Cheeks of same 

 width throughout, most elevated next to the glabella. Surface 

 smooth. 



Middle Cambric (Acadian) of St. John group of New Bruns- 

 wick. 



