1881.] NATURAL SCIENCES OF PHILADELPHIA. 105 



aperture, analagous to a varix, is formed — and this repeated as 

 the animal advances towards maturity, imparts to the shell its 

 special feature. It will be readily seen by this, that any of these 

 forms, scattered or distributed over a wide region in northerly or 

 extremely elevated stations, where the season of cold reaches a 

 maximum, against which protection must be sought by hiberna- 

 tion, might in some of their colonies be subjected to such condi- 

 tions, and hibernation be the only protection, as in the land snails 

 of arid regions, against seasons of excessive drought, and in other 

 regions against the cold of winter ; and bulged or varicose varia- 

 tion be produced in a perfectly simple way, that is, in harmony 

 with or through the operation of a general law ; and this variation 

 be perpetuated for some time in colonies migrating from such 

 stations to a more genial habitat ; until after awhile, some of the 

 descendants of these varicose ancestors reach places where hiber- 

 nation is unnecessary by reason of a permanency or mean of con- 

 ditions — temperature, suppl}' and quality of water being in 

 equilibrium with the usual requirements of these animals — and the 

 ordinary smooth, evenly-grown shells again prevail through re- 

 version to the original form. 



To return to the groups, as above, the Covington Pond speci- 

 mens referred to in part first, connect said groups, being what 

 may be called " trivolvis^ with variations " — that species or 

 general form being, through its plasticit}', the connecting link. 



Still another aspect of variation is shown in Ingersoll's ^ P. 

 plexata, from St. Maiy's Lake, Antelope Park, Colorado. Here 

 we have a variation not unusual in the various Planorbes, and 

 not confined to any of the larger species, that of irregularity in 

 winding, as if through extreme torsion the coil cockled ; the 

 whorls twisting off the line or plane of volution. P. plexata is 

 an eccentrically coiled trivolvis, the deviation from plane of volu- 

 tion having somewhat of regularity of occurrence, and not im- 

 probably owing to the same cause as that to which I attribute the 

 bulging in the glabratus form, namel}^ — to recurring seasons of 

 hibernation and activity, when the new growth hardly makes a 

 " good joint " as a mechanic might say, with the edges of the 

 previous mouth ; the heavy water plants at the bottom of the 

 lake described by Mr. Ingersoll, quite likely perform a part, in 

 causing or contributing to the irregular winding of the shell at 



* See Hayden's Reports Teri-itorial Surveys, 1874, p. 402. 



