52 THE NAUTILUS. 



foothold and continue to thrive locally until the next cycle of mois- 

 ture, and thus in a few hundred years, perhaps extend their range a 

 long way, and in a few thousand years cross a State or two. I have 

 some interesting mollusk records bearing upon the question of such 

 fluctuations, not yet published. So the mere fact of intervening dis- 

 tance is not a fatal objection to considering the Wyoming shells 

 identical with the Grand Canyon form, if you do not think the 

 difference in the keel is sufficient to separate them, of which I am 

 doubtful myself. * * * * Professor Cockerell was just in the Museum 

 and we went over the extremitatis material together, and conclude 

 that in the unerocled shells there is no essential difference between 

 the Wyoming specimens and those from the Grand Canyon, so I am 

 labelling them all extremitatis." l 



r- 



OKEOHELIX COOPERI (W. G. B.). 



Trapper's Creek, 7 miles east of Shell, White Creek Canyon. 

 Shell Creek Canyon, 10 miles northeast of Shell. 2 miles south of 

 Anchor on the north slope of mountain at its foot, among pines and 

 underbrush, on Owl Creek. This place is about 60 miles south of 

 Shell Creek. 



OREOHELIX PTGM.EA n. sp. Plate III, figs. 10, 11, 12. 



The shell is related to 0. cooperi, from which it differs in being 

 smaller, with a narrower umbilicus, much more convex whorls, and 

 rougher sculpture. The spire is convexly conic. Embryonic two 

 whorls moderately convex, finely and weakly striate, with a few dis- 

 tinct spirals on the latter part near the periphery. Post embryonic 

 whorls increase very slowly and are very convex, the convexity 

 greater above the middle of each, with sculpture of strong, irregular 

 wrinkles along growth-lines and moderate or very weak spiral im- 

 pressed lines. On the last whorl, which is very convex throughout, 

 the sculpture is strongest above, but continues upon the base. Suture 

 deeply impressed. Umbilicus small, its width contained five to six 

 times in that of the shell. Peristome thin, forming | to |- of a circle. 

 The spire is flesh-colored when worn, fleshy-brown with whitish 

 streaks in young shells; the last 1 or l whorls are impure white 

 with fleshy streaks ; marked with a deep brown band below the 



1 Junius Henderson in letter, Nov. 15, 1912. 



