8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 93 



At first glance this species, which was made the genotype of 

 Amccephalus, is quite distinct from Alokistocare. Closer examination 

 shows conclusively that it is congeneric with A. subcoronatum and 

 must therefore be referred to it as the older genus. Because the speci- 

 mens are considerably flattened, A. piochensis appears somewhat dif- 

 ferent from A. subcoronatum, but if several specimens are carefully 

 examined and variations due to preservation noted, generic distinc- 

 tions are wanting. 



Description. — Entire shield tapers from a wide cephalon to a small 

 pygidium ; but because of the fewer thoracic segments, this feature 

 is less pronounced than in other, longer, undescribed species. 



Cephalon semicircular in outline and apparently of moderate con- 

 vexity. Cranidium about equally broad as long. Anterior facial su- 

 ture diverges about 14°, to rather square anterior corners; intra- 

 marginal possibly one-half the distance to the center ; posterior por- 

 tion diverges rapidly, forming long, narrow postero-lateral limbs. 

 Glabella well defined by dorsal furrow, which, as usual, is shallower 

 across the front and anterior to the eye lines ; tapers considerably and 

 is a little over half as long as the cephalon. Three pairs of glabellar 

 furrows traceable in some specimens. Occipital furrow shallow in the 

 middle portion, separating a ring of even width. Brim wide, strongly 

 striated vertically, anterior to the eye lines ; a slight median boss 

 usually discernible. When the free cheeks are retained, a narrow rim 

 is apt to be present. In most individuals the doublure is pressed 

 through so that its inner edge shows on the upper side as a ridge, with 

 the consequence that many photographs seem to show a broad rim ; 

 doublure much wider in the center than toward the anterior angles, 

 owing to the intramarginal course of the facial suture on the under 

 surface. Fixed cheeks wide, eyes small, strongly bowed ; eye ridges 

 strong and, when undistorted. apparently fairly straight. Free cheeks 

 also wide, with a concave border ; sometimes with a narrow rim ; 

 genal spines concave and sharply pointed. 



Thorax with 19 segments. Pleura straight ; fulcrum far out ; fur- 

 rows in a central position extending to the fulcrum. 



Pygidium small, with well-defined axis extending its full length. 

 Pleura well fused, with pleural furrows showing only faintly on cer- 

 tain moulds. 



Comparisons. — Confusion of this species with described forms is 

 not likely because of its broad brim and widely placed fulcrum. 



Middle Cambrian, Chisholm ; (loc. 31) 3 miles northwest of Pioche, 

 Nevada. 



Lectotypc and paratypes. — U.S.N.M. no. 15434b. 



