446 GEOLOGY OF THE YELLOWSTONE NATIONAL PABK. 



Formation and locality: Upper Cambrian, Gallatin limestone, Crow- 

 foot section, Gallatin Range, Yellowstone National Park; Hamburg shale 

 near Hamburg mine, Eureka district, Nevada. A variety also occurs in 

 the Secret Canyon shale 1,200 feet below the Hamburg shale. 



DICELLOMUS Hall. 



Dicellomus Hall, 1873 : Twenty- third Ann. Rept. New York State Cab. Nat. Hist., p. 246. 

 Obolella Hall, 1892: Pal. New York, Vol. VIII, Pt. I, p. 72. 



When proposing that the genus Dicellomus include Obolella polita, Pro- 

 fessor Hall stated that the grooving or emargination ot the apices of both 

 valves and the thickening of the edges of the shell on each side below the 

 apex, together with the form and character of the muscular impressions, 

 would separate the species from Obolella. Again, in 1892, Messrs. Hall and 

 Clarke gave a fuller description of Dicellomus politus, but owing to the poor 

 character of the material, he did not feel confident that it should be recog- 

 nized as generically distinct from Obolella chromatica. Material now in the 

 collections of the Geological Survey clearly shows that Professor Hall's 

 provisional conclusion was correct, and that Dicellomus politus is generically 

 distinct from Obolella chromatica. The generic characters are also finely 

 shown by specimens of Dicellomus nana (M. and H.) from the Little Rocky 

 Mountains of Montana, and also by the interior of the ventral valve, figured 

 by Meek and Hayden. The cast of the interior of the ventral valve 

 is shown by fig. 3c, PI. LX, and the interior of the ventral valve from 

 the Black Hills by fig. M, PI. LX. These will be found to differ from the 

 illustrations of Obolella polita given by Professor Hall, 1 but the material 

 from which the figures were drawn was poor, and, to a certain extent, the 

 drawings are somewhat constructive, as stated by Professor Hall. 2 From 

 the specimens before me, as shown by figs. 4, 4a, PI. LX, it is clear that 

 Dicellomus politus and D. nanus are congeneric. Further illustrations of 

 the characters of D. politus will be given in a review of the Cambrian 

 Brachiopoda. 



' Sixteenth Ami. Rept. New York State Cab. Nat. Hist., 1863, PI. VI, tigs. 20, 21. 

 »Pal. New York, Vol. VIII, PI. 1, p. 72. 



