100 PROCEEDINOS OF THE ACADEMY OF [1881. 



apex the less depressed, and therefore the latter is more easily 

 discerned. Another point too important to be overlooked, is the 

 form of the tube as exliibited in a transverse or cross section ; 

 whether circular, semilunar, or horizontally or perpendicularly 

 ovate. 



Some of the larger species have tubes, which, in cross section, 

 are of the latter shape, hence the physoid aspect of the aper- 

 ture both in adult and embryonic specimens. 



Since the foregoing was written, the shells which appear in the 

 list appended hereto as from Lake Simcoe, came to hand. The 

 lot embraced three species of Planorhis. Several specimens (of 

 rather small size) of P. trivolvis, are of the western P. corpu- 

 lentus character, and sinistral. Twenty-five specimens of P. 

 campanulatus, Say, are also sinistral as described by that author, 

 and three specimens of P. bicarinatus are dextral, though des- 

 cribed as sinistral b}' Say. Of the smaller American species 

 glanced at by me in the course of investigation, I find P. vermic- 

 ularis^ from Utah Lake, U. T., is sometimes dextral. 



From what is presented above it will, I think, be admitted that 

 some species of Planorbes have shells whose structure is in har- 

 mony with the sinistral characters of the anatomy, as might 

 reasonably be presumed, and it is not unlikely that such will 

 prove to be the rule and not the exception, when an extended and 

 critical examination of the whole group has been made. 



I do not propose, at this time, to inquire into the origin of the 

 related forms referred to in this paper ; but the suggestions, which 

 have incidentally occurred in, or grown out of the consideration of 

 the simpler points discussed, impress me as inviting investigation. 



The following species of Planorhis from the localities stated, 

 have been especially examined in connection herewith. 



* P. trivolois, Erie Canal, N. Y. 



* " Tinker's Creek, Lake Co., Willoughby, O 



* _ " Foot's Pond, Woodburn, near Cincinnati 



Ohio, 



* " Lake Winnebago, Wisconsin. 



* " Wabash River, Posey Co., Indiana. 



* '' Covington, Kentucky, 

 f " Washoe Lake, Nevada. 



Note. — I am indebted to the courtesy of the Smithsonian Institution 

 for all of the figures herein, except 10, 11, 14 and 15, which are original. 



