Page:Walcott Cambrian Geology and Paleontology II.djvu/277

 specimens with several segments united show them to be much like those of Molaria spinifera (pl. 29, fig. 2).

The abdomen (pygidium) is apparently semicircular with a rudely defined median lobe and two or three segments outlined on it.

Appendages.—Head. A portion of what may be an antenna projects from beneath the right anterior margin; from near the left posterolateral angle a large four-jointed appendage extends backward. I assume that this may be the outer portion of the large posterior appendage (maxilla) of the head.

Thorax. Traces of several slender-jointed thoracic legs project from beneath the anterior segments and back of these on the right side more or less of six legs have been pushed out from beneath the dorsal shield; these are composed of three to four long, slender joints; fragments of the three proximal joints indicate that they are shorter and larger and that they have a fringe of fine setæ. Indications of a branchial lobe (gill) are seen in two specimens where the legs are not preserved. This is often the case both among the Merostomata (pl. 29, fig. 3) and Trilobita (pl. 24, fig. 2).

Two caudal rami project a little distance from beneath the posterior margin of the dorsal shield.

Dimensions.—The only entire specimen has a length of 45 mm. Its other proportions are shown by figure 2, plate 28.

Observations.—I have given the specific name transitans to this species on account of its suggesting a transition between a Merostome-like form, such as Molaria spinifera, and the trilobites (pl. 24, figs. 1 and 2). This is mentioned under remarks on the appendages of the trilobites (p. 191).

The specimen illustrated was found by my son Sidney where we were using dynamite to "sledge" our way into the solid ledge of hard shale as it was back from the action of frost that the most beautifully preserved specimens were found. A few fragments turned up later, and we hope to find more perfect specimens in the future.

Formation and locality.—Middle Cambrian: (35k) Burgess shale member of the Stephen formation (phyllopod bed), 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.

It is highly probable that the new genera Mollisonia and Tontoia will come within the family Microdiscidæ Coquin, 1896.