THE NAUTILUS. 65 



During the hot months, from about July 1st to October 1st 

 usually, the stream is dry, or water is to be found only in an 

 occasional pool ; during the rest of the year the water flows to 

 a depth of a few inches. The bed is scoured out of limestone 

 (Austin Chalk) and hafi for much of its length a solid flat rock 

 bottom. The banks of the creek have in general a gradual 

 slope. In time of flood and during heavy rains, this stream 

 rises very rapidly and quickly becomes bank-full, so that the 

 water rushes down in a torrent, the roar of which may at times 

 be heard for a distance of some blocks. The fall of the creek is 

 considerable, being about 75 feet in two miles from Twenty- 

 seventh Street to the Colorado, and this fall in connection with 

 the shape of the bed gives to the current in times of flood a 

 tremendous force. 



During the first two years of this observation, 1912-1913, the 

 snail population of the creek in its middle stretches was dense. 

 There were in particular two species very thickly represented, 

 Planorbis lentus and Physa halei. So numerous were they that 

 one could in a few moments within a very few feet gather a pint 

 of either kind. Wherever a little ripple or a tiny waterfall oc- 

 curred were many snails oriented in relation to the current, their 

 heads pointing into it. Elsewhere in the more quiet water they 

 were also to be found, but in less numbers. These conditions 

 obtained especially in the early spring; as the breeding season, 

 which in that latitude extends over half the year, passes by, the 

 snails of course become much less numerous. 



It is to be noted that previous to the time when the snails 

 had become so abundant, there had been no heavy rains of suf- 

 ficient importance to be recorded since 1908. Excessive rains 

 occurred in May 1908, November and May 1907, June and 

 March 1905, May and April 1904, July and February 1903, 

 July and November 1902. There was a very severe flood in the 

 creek in April 1900. It will be seen that between the time 

 when my observations began and the last excessive rain con- 

 siderable time had elapsed and the snails had had the oppor- 

 tunity to reinstate themselves in the creek, assuming that they 

 had suffered in those floods as they have done in the later ones. 



In the fall of 1913 there were two floods of unusual propor- 



