NO. 2 MIDDLE CAMBRIAN HOLOTHURIANS AND MEDUS.?-: 5 1 



of the proboscis-like lower portion of the body. In contrast the 

 Middle Cambrian type had a true medusa-like umbrella ; concentric 

 subumbrella muscle band ; spiral subhorizontal alimentary canal, 

 with mouth and anus off to one side of the center; and, judging 

 from what is known of the umbrella-like body, opening at the ven- 

 tral surface. The water vascular system indicated by the central 

 ring (cr) and numerous radiating canals (re) (pi. 8, fig. 3 ; and pi. 9, 

 figs. I and 5), also serves to give the Cambrian form a character 

 unlike that of Pclagothuna. 



That the mouth and anus should open on the ventral surface is 

 not unexpected, and the development of the radiate structure of the 

 smaller canal system is also the result of the animal's gradually 

 shifting the relations of its parts to each other, in the course of ad- 

 justment to its pelagic habitat. 



The finding of a true medusa at the same locality, Peytoia nath- 

 orsti (pi. 8, figs. I and 2), also many free swimming crustaceans, 

 indicates that the environment and food supply were favorable to a 

 free swimming holothurian. The presence at the same locality of 

 tvpical holothurians is very instructive, although they occur three 

 to four feet lower down in the shales. 



The specific name is given in honor of Dr. H. Ludwig, who has 

 done such splendid work on the holothurians dredged by the Alba- 

 tross.' 



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



^ Mem. Mus. Comp. Zool., Vol. 17, 1894, No. 3, pp. 1-184, pis. 1-19. 



" Burgess Shale 



This name is proposed as a geographic name for a shale to which the term 

 of Ogygopsis shale was given in 1908 (Smithsonian Miscellaneous Collections, 

 ^'o\. 53, p. 210). It is proposed to call it the Burgess shale of the Stephen 

 formation. 



Type locality. — Burgess Pass east of Mount Burgess and on the west slope 

 of Mount Field and the ridge extending to Wapta Peak. About 3000 feet 

 above and from three to live miles on the trail from the town of Field on 

 the Canadian Pacific Railway, British Columbia, Canada. The Burgess for- 

 mation occurs to the southward across the Kicking Horse Canyon in the 

 side of Mount Stephen. 



Derivation. — From Burgess Pass, the type locality. 



Character. — Argillaceous, calcareous, and silico-argillaceous shales. 



Thickness. — On the west slope of Mount Field, 420 feet; on the northwest 

 slope of Mount Stephen, about 150 feet. 



Stratigraphic position. — Above thin bedded, dark gray, and bluish-black 

 limestones of the Stephen formation, and beneath a thin bedded limestone 



