104 PROCEEDINGS OF THE ACADEMY OF [1881. 



P. hicarinatus, the cone of which is less robust than that of P. 

 amnion oi' P. Traskii, being in that respect nearer to the typical 

 trivolvis, exhibits the culmination of the carinated or keeled 

 character of the second group, and appears to mark tlie limit in 

 this direction, having reached what may be termed a permanent 

 point. This species is usually quite persistent as to plane of 

 coil ; though in Binney it is reported from a single station as far 

 south as "Northern Georgia" — it seems to prefer northerly 

 regions. 



It is apparently of rare occurrence west of the Rocky Moun- 

 tains. Mr. Hemphill informed me that he detected two or three 

 individuals at Antioch, California, a station peculiar in its enviro- 

 mental characters, being at a point where the Sacramento and 

 San Joaquin Rivers meet and unite the drainage waters of the 

 two great valleys of the same names; mingling in combined 

 volume their fresh water with the salt tidal-water from San Fran- 

 cisco Bay. Other forms are sometimes found at this point ; they 

 seem unable to obtain a foothold or to establish a permanent 

 colony or settlement. The region is one of marshes, which sus- 

 tain a rank growth of coarse vegetation, especially what is known 

 as tales, whicli sift, as it were, the waters, and hold for a time 

 forms which, during the great floods of excessively wet winters, 

 are swept from their native haunts through the submersion or 

 overflow of the ponds, lakes and streams of a vast interior 

 region. 



Thus Mr. Carlton^ found a few juvenile specimens of Carinifex 

 here in May, 1869, which, like Mr. Hemphill's specimens of 

 P. bicarinatus, had never before nor, so far as I can learn, have 

 never since been reported from this place nor any point in the ad- 

 joining region. In fact, the only habitat west of the Rocky Moun- 

 tains, I believe, from which this Planorbis is reported on good 

 authority, is Oregon ; I have specimens from Portland, collected 

 by Mr. Hemphill. 



A frequent aspect of variation in the forms falling within the 

 first gi'oup, is that of occasional bulgings or swellings, as seen in 

 P. glabratus, Say, and P. tuinens, Cpr., suggesting periodicity in 

 growth, or rather periods of hibernation or rest, and periods of 

 activit}', at the termination of which a mouth or expansion of the 



1 Proc. Cal. Acad., Vol. IV, p. 50, 



