THK NAUTILUS. 137 



recorded it from Durango. A thriving colony was found just north 

 of Morrison (Rohvver). 



I visited this colony on September 23 with Mr. Rohwer, and find 

 it the most remarkable yet discovered east of the Front Range. 

 Though scattered snails were found over the whole slope above, they 

 were abundant only under three small groups of skunk-bush (Rhus 

 trilobata Nutt.), where the soil contained many fragments of lime- 

 stone which had slid down from up the slope, the aggregate area of 

 the groups covering less than 100 square feet. Under one root were 

 46 specimens clinging one to another in a cluster. As they average 

 over three-fourths of an inch in major diameter, one may imagine the 

 size of the cluster. On the under surface of a board on an area of 

 48 square inches, 98 were found, making almost two layers. Under 

 a rock o by 6 inches were 23. Within a radius of 8 inches were 80. 

 These were all alive. In a short time we collected 953 live ones. 

 Though the weather had been exceedingly dry and warm for weeks, 

 they were scarcely buried, the soil being too hard for them to burrow 

 into had they wished to, except an inch or so of debris and leaves on 

 the surface. In digging we found none deeper than 3 or 4 inches, 

 none below the loose debris, mostly barely covered and many ex- 

 posed, but all dormant. We carried them home in a box and the 

 combined moisture of the mass seemed sufficient to awaken them, so 

 that the next day they were all active and their shells moist. This 

 may suggest the reason for clustering in this semi-arid region. 



With this exception, the finest colony yet reported is one I found 

 last August north of the river a mile below Steamboat Springs, on a 

 steep, dry, south-facing slope of lower Mancos Cretaceous shales and 

 limestone. Time was too limited for much collecting. At the first 

 point of contact with the colony 0. cooperi predominated, while less 

 than half a mile to the west depressa predominated, thus : At the 

 west 112 depressa and 16 cooperi collected ; at the east 106 cooperi 

 and 77 depressa. Hot, dry weather, live snails well concealed. A 

 considerable proportion of both species were quite dark and many 

 showed the color characters of 0. s. albofasciata Hemphill, the upper 

 dark band extending to the suture, the lower one to the umbilicus, 

 with a white band on the periphery, but other examples showed com- 

 plete gradation into the typical forms. As I have found this varia- 

 tion and gradation in other colonies of each of the two species, and 

 finding none that cannot be referred to one or the other, the record 



