190 



PROCEEDINGS OF THE ACADEMY OF 



[March, 



adults measuring 12 to 15 mm. diam., as in the head of Quaking Asp 

 Canyon. 



umbilicus 4.5 mm. (Station 91). 



3.8 

 6.3 

 5.8 

 6.0 

 2.9 



98) 



") 



84) 



(11) At Station 100, Kaibab-Powell Saddle, 6,700 feet, the shells 

 resemble those figured from Station 98, but are smaller, diam. 17.5 

 to 21 mm. 



(12) Off the north end of Powell Plateau, at Station 17 (PI. XIV, 

 figs. 1-4), there is wide color-variation in the colony, the following 

 f orms occurring : 



Figs. 1, 3. Typical two-banded form, diam. 18 to 21 mm. 



Fig. 2. Upper band obsolete, the lower are weak same sizes. 



Fig. 4. White, with greenish, translucent bands, diam 19 mm. 



(13) In other stations at the northern end of Powell Plateau, 13, 

 14, 15 (but not Station 16), and Station 105, near Oak Springs, the 

 prevalent form (PL XIV, figs. 6, 7, 8) Station 15, 6,500 feet) is rather 

 less depressed than typical depressa, though the depressed form also 

 occurs (PI. XIV, fig. 5, same station). At Stations 13, 15 and 105 

 some examples are not distinguishable from 0. cooperi (PI. XIV, fig. 9, 

 Station 15; fig. 23, Station 105); yet some examples seem to connect 

 with normal depressa. At Station 18, at 6,700 feet, two adult shells 

 (PL XIV, figs. 10, 11) are of the cooperi form. 



Fig. 10. Alt. 12.0, diam. 18.7, width of umbilicus 4.5 mm. 

 " 11. " 11.9, " 18.0, " " 4.0 " 



The material from these stations (13, 15, IS) is too scanty to decide 

 with certainty whether the series is divisible into cooperi and depressa 

 or whether the globose specimens are inextricably connected with 

 the depressa stock of the region. Except at Station IS, the cooperi 

 form occurs with undoubted depressa, as at Station 105, where figs. 

 22 and 23 of PL XIV occurred together. 



(14). The west side of Powell Plateau, Stations 25, 24. 22, 21, 20, 

 19, and Station 16 at the north end, have a somewhat different race. 

 There are typical two-banded depressa (PL XIV, figs. 13, Station 16) 

 and also specimens lacking one or both bands (PL XIV, figs. 14, 15, 

 Station 16, 6,700 feet; fig. 16, Station 19). 



The specimens vary from quite large, diam. 25, to medium size, 



