NO. 6 MIDDLE CAMBRIAN BRANCHIOPODA, ETC. I95 



specimens with several segments united show them to be much like 

 those of Malaria spinifera (pi. 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 pro- 

 jects 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 setse. 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 (pi. 

 29, fig. 3) and Trilobita (pi. 24, fig. 2). 



Two caudal rami project a little distance from beneath the pos- 

 terior 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 (pi. 24, figs. 

 I 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, 



Order HYPOPARIA Beecher 

 Family (undetermined) 



It is highly probable that the new genera Mollisonia and Tontoia 

 will come within the family Microdiscidse Coquin, 1896. 



