THE NAUTILUS. 51 



iast whorl far less swollen above. In 0. hemphilli all post embryonic 

 whorls are more convex above. Moreover, 0. hemphilli, according 

 to Mr. Binney, has well-developed side-cusps on central and lateral 

 teeth. These are entirely wanting in 0. yavapai and its varieties, 

 in numerous specimens I have examined, including some from Shell 

 Creek Canyon. 



The Oreohelices are divisible into two groups by certain peculiar- 

 ities of the soft anatomy. One of these groups has hitherto been 

 found only in Arizona, and comprised 0. barbata of the Chiricahua 

 Mts., and 0. yavapai of central Arizona and New Mexico, with 

 several subspecies in the Grand Canyon. 1 0. y. extremitatis lives 

 just below the rim of the Canyon. On comparison with the speci- 

 mens from Wyoming, no tangible difference, either in the shell or 

 the soft anatomy could be found. Both vary somewhat in the prom- 

 inence of the peripheral keel, but in the Wyoming specimens it is 

 usually a trifle more prominent than in the average extremitatis from 

 Arizona, the most strongly keeled " subfossil " shells reminding one 

 of 0. chiricahuana. On comparing a large number the difference is 

 minimized, selected extremitatis from the type locality having the 

 keel as strong as in any of the Wyoming lot. 



The extension of this form northward is unexpected, the whole 

 width of Utah and Colorado as well as most of Wyoming lying 

 between its northern and southern areas. Mr. Henderson writes : 



" I suppose little is yet really known of the molluscan fauna of 

 the region intervening between the Grand Canyon and Wyoming. 

 I find but little published literature on Wyoming shells, and do not 

 know of much collecting having been done in eastern Utah or west- 

 ern Colorado. It is true that at present there are great stretches of 

 country unfavorable to land snails in that region, and equally true 

 that semi-arid conditions have prevailed in the southwest for a long 

 time probably since Pliocene time. Nevertheless, there are sev- 

 eral reasons for believing that during that long period of semi-aridity 

 there has been more or less fluctuation within narrow limits. Dur- 

 ing each cycle of increased moisture favorable conditions would reach 

 greater extent, and if the snails spread with the increase of favor- 

 able territory, there would be small areas where they could obtain a 



'See "Mollusca of the Southwestern States V," 1911, where the Oreohelices 

 of the Grand Canyon are figured. 



