96 PROCEEDINOS OF THE ACADEMY OP [1881. 



species; also with other species of PJiysa, like P. hvmerom 

 (fig. 7), P. ancillaria (fig. 8), P. ampullacea (fig. 9', etc., ct sic de 

 similihus. 



We shall, however, find more satisfactory testimony on the 

 sinistral point by analyzing the apical characters. 



If, as in other shells, we consider that to be the upper end or 

 spire in which we can follow tlie volutions through the entire shell 

 from tip or nucleus to the ultimate or basal whorl and mouth — 

 then it is impossible to avoid the conclusion that some species of 

 Planorbis have sinistral shells. In some of the smaller forms, 

 like P. vortex^ P. conforfus^ P. glaber, P. carinatus^ and P. spir- 

 orbis, all of which are British species, and in which the whorls 

 are (in comparison with larger American forms) quite evenly 

 coiled and in which also, the increase in size of whorl is quite 

 gradual, the difference between the two sides of the shell, apical 

 and umbilical, is not as readily perceived. The largest British 

 species, P. corneus, confirms my view, being sinistral. The rapid 

 enlargement of the whorls in some of the West American species 

 is in marked contrast with even the shells of P. corneus of the 

 same diameter ; the height of the latter as compared to P. corjni- 

 lentus being as -31 to "54, while the length of the aperture is in 

 still greater contrast, being as -42 to -YG. These measurements 

 were made from average-sized perfect specimens of both species 

 placed apex up, with the mouth to the left, 



It is easy to perceive that in those forms where the ratio of 

 increase is great as between the last whorl and the preceding Arolu- 

 tion and so on, whorl compared with whorl, through the whole, as 

 for instance in P. ammon^ that the depression of both spire and 

 umbilicus is most marked ; but nevertheless the umbilicus is the 

 more profound as may easily be proved by counting the volutions 

 first on one side, and then on the other. 



Again, if additional proof is wanted, take any one of the larger 

 forms^ of the species herein named, and carefully, by degrees, burn 

 off the under side (which can easily be done, by pouring a little 



been suspected.— ^rt«-rt7s N. T. Lyceum Nat. Hist., Vol. IX, p. 35G ; foot- 

 note. 



1 As distorted individuals, with the volutions exceedingly irregular, are 

 of frequent occurrence, in making the test suggested, such monstrosities 

 should be rejected, as they would as often unduly favor one side of the 

 question as the other. 



