NO. 6 MIDDLE CAMBRIAN BRANCHIOPODA. ETC. 169 



Compared with recent species of the Anostraca, Opabinia regalis 

 is from three to four times as large, since the former average from 

 10 to 24 mm. in length. 



Female. — ^There are two associated specimens that I have referred 

 to the female of this species. One has a length of 6i mm. and the 

 other of 52 mm., exclusive of any frontal appendages. The female 

 differs from the male in having two slender caudal appendages or 

 rami ; and in having a slender bifid frontal appendage instead of the 

 strong appendage of the male. The character of the frontal append- 

 age is more or less doubtful as it is turned under and back on the side 

 of the body. I hope that we will find in the collections of the summer 

 of 19 12 specimens that will add much to our knowledge of all parts 

 of both the male and female of this species. 



Observations. — Compared with recent forms Opabinia regalis has 

 many outward characters of Thamnocephalits platyurus Packard.^ 

 The proboscis, form of head, body segments, and expanded terminal 

 segment or telson are very suggestive of Thamnocephaliis. So far 

 as can be determined the structure of the thoracic legs is essentially 

 similar, but this of course is subject to revision. After flattening 

 specimens of Branchinecta and Branchipus between plates of glass 

 and studying them, I am greatly surprised that any distinct char- 

 acters of the appendages are preserved in the fossils in a recognizable 

 condition. 



The frontal appendage is referred to as the proboscis. It is united 

 directly with the front of the head ; it was flexible and provided with 

 a central canal that may be traced from its base out to the expanded 

 end, which has a circle of small, curved claw-like spines attached to 

 it. The function of the proboscis and its terminal spines is unknown ; 

 it appears to be adapted to the gathering of food and conveying it to 

 a mouth beneath the head, but it was probably used by the male to 

 seize the female. 



If we consider the appendage-bearing somites as the thorax, the 

 abdomen is confined to the one elongated expanded somite I have 

 referred to as the telson. This does not show evidence of segmen- 

 tation unless there is a post-anal plate, which is very doubtful. None 

 of the specimens of the male show any traces of caudal appendages. 



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

 member of the Stephen formation, on the west slope of the ridge 

 between Mount Field and Wapta Peak, one mile (1.6 km.) northeast 

 of Burgess Pass, above Field, British Columbia. 



*U. S. Geog. and Geol. Surv. Territories, 12th Ann. Rept., Pt. i, 1883, pp. 

 353-355, text fig. 23. 



