Page:Walcott Cambrian Geology and Paleontology II.djvu/251

 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 61 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 appendage 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 1912 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 Thamnocephalus platyurus Packard. The proboscis, form of head, body segments, and expanded terminal segment or telson are very suggestive of Thamnocephalus. 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 characters 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 segmentation 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.