NO. 6 InIIDDLE CAMBRIAN BRANCHIOPODA, ETC. I99 



TONTOIA KWAGUNTENSIS, new species 



Plate 24, fig. 4 



This unusual form has a convex dorsal shield divided into a 

 cephalon, thorax, and pygidium. The cephalon has a narrow, raised 

 margin with the lateral third on each side of the cephalon rising 

 rapidly to the elevated central section. The central section is slightly 

 flattened and has a sharp median ridge extending from the anterior 

 end back to the posterior margin. A similar ridge crosses each tho- 

 racic segment and extends back on the pygidium about one-fourth its 

 length. 



The segmentation of the cephalon is suggested by two narrow 

 ridges crossing .the right lateral space beneath the elevated central 

 portion. 



There is a suggestion of the presence of eyes on the posterior 

 outer edge of the elevated central portion of the cephalon. 



The thorax is divided into four segments by rounded, elevated, 

 narrow ridges as shown by figure 4. 



The pygidium is shorter and smaller than the cephalon, but I think 

 this may be owing to the breaking away of the margin. 



There is only one specimen and that is a matrix in a fine, hard 

 sandstone. Trails that appear to have been made by this or a 

 similar form occur on the surface of several of the layers of sand- 

 stone adjoining the one on which the specimen illustrated occurred. 



I am not at all sure that this species should be placed with the 

 trilobites, but with such forms as those illustrated by figures 3 and 5 

 at the same geological horizon it seems best to classify it with them 

 for the present. 



The total length of the specimen illustrated is .25 mm. Other 

 proportions are shown by figure 4. 



Formation and locality. — Middle Cambrian: (y^) Tonto sand- 

 stone, upper portion, Kwagunt Valley, Grand Canyon of the Colo- 

 rado, Northern Arizona. 



Sub-Class MEROSTOMATA 

 Order AGLASPINA, new order 

 Body elongate, transversely trilobed. Cephalo-thorax with or 

 without sessile eyes ; on the ventral side it has an epistoma and five 

 .pairs of movable appendages. 



Thorax with 8 to 11 segments, each of which has a pair of jointed 

 appendages. Abdomen with i to 3 segments. 

 One family, Aglaspidge Clarke. 



