NO. 2 MIDDLE CAMBRIAN HOLOTHURIANS AND MEDUSA 57 



had considerable substance to it, as it has left a very clear impression 

 and the lobes still retain a slight convexity. 



Among fossil medusse some of the many lobed specimens of Laotira 

 Cambria Walcott ^ might be compared with this species on account 

 of the numerous lobes of the umbrella disk, but beyond that there 

 are no points in common between them. The large quadrate opening 

 of the subumbrella may be compared with the quadrate mouth of 

 Mednsina costata (Torell),^ but here the comparison ends, as the 

 genital hollows in P. nathorsti are not preserved and the subumbrella 

 of M. costata is not well defined. 



It is hoped that during the field season of 191 1 more perfect speci- 

 mens of P. nathorsti may be found. 



The associated fossils are Eldonia liidzvigi, Ptychoparia cordillerce, 

 Neolenus serratus, Sidneyia inexpectans, and numerous undescribed 

 annelids and phyllopod crustaceans. 



The specific name is given in honor of the distinguished Swedish 

 paleontologist, Dr. A. G. Nathorst. 



Formation and locality. — Middle Cambrian: (35k) Burgess shale 

 of the Stephen formation ; west slope of ridge between Mount Field 

 and Wapta Peak, one mile northeast of Burgess Pass, above Field 

 on the Canadian Pacific Railway, British Columbia, Canada. 



' Fossil Medusse: Monograph U. S. Geol. Survey, Vol. 30, 1908, pi. 8. 

 ^ Idem, pi. 30, fig. I. 



