Abnormal Fresh-water Shells. 53 



Plakorbis exacutus, Say. 

 Plate I, figs. 1-3. 



The examination of a large series of specimens from the vicinity of 

 Albany shows that this species frequently departs from its normal 

 form. Individuals with expanded and variously modified apertures 

 are not uncommon and one sinistral example has been detected. 



Figure 1 represents an individual in which the upper side of the 

 labrum is expanded. 



Figure 2 represents an individual in which the entire aperture is 

 inflated, especially on the lower side. 



The sinistral specimen (figure 3) has lost nearly all the testaceous 

 characters belonging to the species and is a monstrosity in every par- 

 ticular. It is impossible to determine from external evidence whether 

 it is a case of true sinistrality or one of inverted growth. The volu- 

 tions are of equal convexity on either side and the obliquity of the 

 aperture-is not determinative. The specimen was found in a locality 

 abounding only in this species, and the three specimens here described 

 were selected from among several thousand others, about two per cent 

 of which show some departure from normality, principally in variations 

 in the form of the aperture and elevation of the spire and in intermit- 

 tent growth. 



Valvata tricari;nata, Say. 

 Plate I, fig. 9. 



The volutions of the specimen are free except at the apex. This va- 

 riation in this species has been recorded by several observers and is not 

 extremely rare, although this is the only specimen which has been 

 found in the vicinity of Albany. 



Gtllia altilis, Lea. 

 Plate I, fig. 5. 

 A very remarkable biflexed individual. The shell, for a considerable 

 period of its growth, equal to the formation of the three initial volu- 

 tions, is dextral and of the usual form. The spiral then changes its 

 direction ; the apex becomes partially inverted and the last volution is 

 sinistral. This is the only example of a heterospiral growth that is 

 known to,me and cannot be satisfactorily accounted for from the ap- 

 pearance of the shell alone. An examination of the anatomy of the 

 animal might have revealed the cause of this reversion of grow T th. The 

 initial point of the operculum being nearer to the apex of the shell, 

 indicates that the growth was inverted during the formation of the 

 last volution, and suggests, as a possible explanation, the action of 

 gravitation on an animal too weak to hold the shell on its dorsum. 



