NO. 6 MIDDLE CAMBRIAN BRANCHIOPODA, ETC. I97 



The thorax has seven transverse segments outlined, the anterior 

 of which is somewhat narrower than the others ; the segments ter- 

 minate in blunt, falcate, slightly furrowed ends that overlap on the 

 next posterior segment within the posterior curve of the free portion 

 of the end of the segment. A lobation of the thorax is indicated by 

 the presence of a shallow, elongate depression on seven of the seg- 

 ments one-third of the distance across from the left side ; if similar 

 depressions existed on the right side they have been obscured by the 

 flattening of the test in the shale. 



The pygidium curves outward from its anterior margin, and the 

 rounded outline extends along the sides and posterior margin ; it 

 shows traces of an anterior transverse furrow indicating a segment. 



Dimensions. mm. 



Length of dorsal shield 48.0 



Width of cephalon 17.0 



Length of cephalon 13.5 



Greatest width of thorax 18.0 



Length of thorax 24.0 



Width of pygidium 15.0 



Length of pygidium 12.5 



Observations. — The only known specimen of this unique fossil 

 was found on a large slab of calcareous shale in association with 

 Bathyuriscus rotundatus (Rominger), Ogygopsis klotzi (Rominger), 

 and Anomalocaris canadensis Whiteaves. The impression of the thin 

 test is very clear and along the left side some of the test remains as a 

 very thin dark scale. The specimen is in the same condition of 

 preservation as the thin tests of the shields and body of Anomalocaris 

 canadensis and was evidently much thinner and more delicate than 

 the tests of the associated trilobites. 



Formation and locality. — -Middle Cambrian: (14s) Lower por- 

 tion of the Stephen formation, northwest slope of Mount Stephen, 

 above Field, British Columbia, Canada. 



MOLLISONIA GRACILIS, new species 



Plate 24, fig. 5 



Of this species only one specimen is known. The general outline 

 and proportions of the dorsal shield are shown in figure 5. The 

 specimen has been laterally compressed so that the lateral edges of 

 the cephalon, segments, and pygidium are wrinkled and suggest 

 fine spines. 



One of the striking peculiarities is the transverse anterior merging 

 of the cephalon into the short blunt spines projecting from it. 



