274 



NORTH AMERICAN INDEX FOSSILS. 



XX. Albertella Walcott. 



Cephalon large, semicircular, with long genal spines. Glabella 

 subquadrangular, with short, lateral furrows. Fixed cheeks nar- 

 row. Thorax with seven segments, the pleura ending in spines, 



Fig. 1570. Zacanthoides typicalis, 

 view of the type specimen, partly re- 

 stored, X 2. (After Walcott.) 



Fig. 1 57 1. Zacofithoides spinosus, 

 slightly enlarged. (After Walcott.) 



those of the third segment in longer spines; the broad pleural fur- 

 row largely filled by an elongated tubercle. Pygidium large, axis 

 broad, the first anterior or first and second united segments ex- 

 tended into a long spine on each side. 



Differs from Zacanthoides in the spinose extension of the third 

 or fourth thoracic segment and in the presence on the pygidium 

 of only one pair of spines. Cambric. 



48. A. helena Walcott. (Fig. 1572, c.) Cambric. 



Moderately convex. Each ring of the thoracic axis with a 

 small median node near the posterior edge and a low transverse 

 ridge next to the dorsal furrow. (Type of genus.) 



Middle ( ?) Cambric of Montana and western Alberta. 



