190 PROCEEDINGS OF THE ACADEMY OF [March, 
adults measuring 12 to 15 mm. diam., as in the head of Quaking Asp 
Canyon. 
Fig. 22. Alt. 12.0, diam. 19.3, umbilicus 4.5 mm. (Station 91). 
SSW 1) ORE alee cee 1 1 Oss a Serie ne sg) Fy 
BS BIS ho ES Spee Oe aati * Gia fh Eph be OR 
Cn Die ae On wee «| yh eae . BS hw sae Ne 
CEG cae Silesian: yon 43 6.0 « Sai gen 
SO At i ah; al ce US MAA OM Peete ee 221) 7. 
(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 (Pl. XIV, 
figs. 1-4), there is wide color-variation in the colony, the following 
forms 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 (Pl. XIV, fig. 5, same station). At Stations 13, 15 and 105 
some examples are not distinguishable from O. cooperi (Pl. 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 cooperz form. 
Fig. 10. Alt. 12.0, diam. 18.7, width of umbilicus 4.5 mm. 
Beat) aaah SL a Aly ess 
The material from these stations (13, 15, 18) is too scanty to decide 
with certainty whether the series is divisible into coopert and depressa 
or whether the globose specimens are inextricably connected with 
the depressa stock of the region. Except at Station 18, 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, 
