Page:Walcott Cambrian Geology and Paleontology II.djvu/253

. The large opening on the side of the head indicates a large pedunculated eye comparable with that of Opabinia regalis (pl. 28, fig. 1).

Appendages.—Of the head appendages, the antennæ are the best preserved. These are large and composed of several strong joints of which three now show from beneath the carapace; the second of these bears a long slender branch on its inner margin, and the third two branches, one of which is similar to that of the second joint. These two branches appear to be composed of one very long slender joint followed at the end by several very short small joints that curve upward and presumably gave the branches flexible extremities; the third and lower branch has a similar slender proximal joint that at its outer end has three slender, jointed branches. This structure makes a very effective clasper of each of the antennæ. Back of the right antenna are two narrow appendages that may be the ends of one of the third and fourth pairs of head appendages.

The thoracic legs terminate in flat, elongate, broad, lanceolate joints. The terminal joint is about three-fifths the entire length of the leg, and has a fringe of strong setæ on its outer and posterior margin. The condition of preservation is such that the details of structure of the other portions of the leg cannot clearly ba determined.

The size and proportions of the type and only example of the species are shown by the lower specimen of figure 6, plate 31.

Observations.—This is one of the rare species in the collection. The anterior half was found after a dynamite blast and later the matrix showing the posterior portion and part of the anterior was picked out of the débris. Working as we often did with cold rain or snow falling, fragments once lost trace of were rarely recovered.

The large natatory, distal joints of the thoracic legs are much like those of Opabinia regalis (fig. 6, pl. 27), also the large eye. For the present the species is placed in the family Opabinidæ, although I fully realize that the reference is of the most tentative character.

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.

The description of Yohoia tenuis embodies the characters of the genus.

Genotype.—Yohoia tenuis, new species.