86 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 75 



Formation and locality. — Upper Cambrian : (60, 64, 54) Secret 

 Canyon shale ?, Richmond Mine, Adams Hill, and New York 

 Canyon, Eureka District, Nevada. 



Genus ELRATHIA Walcott 



Ehathia Walcott, 1924, Smithsonian Misc. Coll., Vol. 75, No. 2, p. 56. 



Observations. — The genus Elrathia, based on Ptychoparia kingii 

 Meek, will include many of the forms hitherto assigned to Ptycho- 

 paria, which genus was used as a sort of dumping ground for a 

 large series of species that had the general characters formerly 

 assigned to that genus. It is not an easy task to separate them into 

 distinct genera, even though several quite independent groups can 

 be readily determined. Ptychoparia (restricted), so' far as the present 

 study has gone, is possibly absent from American Cambrian beds, 

 and may be confined to the Bohemian Basin. It has much wider fixed 

 cheeks and the sutures diverge much more in front of the eyes than 

 in Elrathia. They also cut the frontal rim in a different manner. T.i 

 Elrathia the suture is intramarginal to the center. This results in 

 a widening of the frontal rim anteriorly. In Ptychoparia, on the 

 other hand, the suture is marginal. Both genera have ocular ridges 

 but that of Ptychoparia is much the stronger. 



Ptychoparia has a very pronounced striation of the forward parts 

 of the head, beginning at the eyes and ocular ridges. Elrathia also 

 possesses a striation, but less pronounced and not beginning so defi- 

 nitely at the ocular ridges, and in some species, it is absent. 



The thoracic segments of the two genera are quite distinct. 

 In Ptychoparia the pleural furrows maintain an even width from 

 their origin at the axis out nearly to the end of the pleura, dividing 

 each segment into an anterior and posterior ridge. In Elrathia the 

 pleural furro^y divides the segment into two unequal parts; the 

 anterior one is narrow next to the axis and widens out toward the 

 outer end ; the posterior one is wide next to the axis and narrows 

 outward, but less rapidly than the anterior one and unites with the 

 latter in forming the blunt terminal spine. 



The pygidium of Ptychoparia shows the same peculiarity as the 

 thorax in that the edges of two segments are fused to form the ridges 

 that appear, on first sight, to be the pleural segments and not as they 

 are — made of the anterior ridge of one, and the posterior ridge of 

 the next. The separating depressions are not between segments, 

 therefore, but pleural furrows. The pygidium of Elrathia has under- 



