Page:Walcott Cambrian Geology and Paleontology II.djvu/97

 had considerable substance to it, as it has left a very clear impression and the lobes still retain a slight convexity.

Among fossil medusæ 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 Medusina 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 1911 more perfect specimens of P. nathorsti may be found.

The associated fossils are Eldonia ludwigi, Ptychoparia cordilleræ, 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.