50 THE NAUTILUS. 



were found living in one respect, at least, under a different condi- 

 tion. Without entering into a general revision of the North Ameri- 

 can Physadce, a task which, sooner or later, will be required, I will 

 state that I regard P. triticea as simply an immature aspect of the 

 almost univei-sally distributed Physa gyrina, and the larger speci- 

 mens collected by me at Auburn, though smaller than the average 

 of adult gyrina, connect the two. 



The conditions under which the Auburn lots occur are as follows: 

 The smaller, whicli would sui'ely be placed with triticea, were col- 

 lected close to springs, where the flow of water at its maximum is 

 small, and its catchment is limited and confined in small hollows. 

 A part of the year these springs must be quite dry, though most of 

 the time yielding a feeble trickling stream. Colonies of Physce 

 became established hereabouts, but when mature, attain a size hardly 

 equal to that of an ordinary P. gyrina, minus the final whorl, and 

 showing the characters that P. gyrina exhibits from the apex down 

 to and inclusive of the penultimate whorl. It is an arrested, un- 

 developed form, equal to P. gyrina adolescent. The larger Auburn 

 specimens, though still exhibiting much of the aspect of triticea, in- 

 dicate, as before implied, their relation to gyrina, and were collected 

 where the flow of water is generally permanent and the quantity 

 much greater than in the other instances, but still where there is 

 not even a permanent pool, and no runway of water that could be 

 called a permanent brook or streamlet. 



The geological character of the region is a slate that is folded 

 and tilted and tipped ; it seems much more favorable to the forma- 

 tion of small springs than to jjermanent brooks and flowing 

 streams, though the latter are occasionally met with, being fed by 

 the melting snows of higher elevations. 



The surface of the pond snails, which authors describe with a 

 nicety that Avould be commendable if it were not so often absurd 

 and embarrassing, varies exceedingly in texture, sculpture and color ; 

 adolescent individuals of the same species or colony may be pellucid, 

 and mature ones opaque ; young shells may have a smooth surface, 

 and old ones exhibit growth lines ; the surface in some may be even, 

 in others malleated, and so on ; and color is modified if the speci- 

 mens, when collected and placed in water, are allowed to remain 

 until nu\ceration has reached a ])utrid stage, when the interior of 

 the shell becomes blackened either by the adherence of decayed 

 matter or by chemical discoloration ; the membi-anaceous composi- 



