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Cincinnati Society of Natural History. 



caused several conchologists to describe the upper and lower 

 surfaces as the right and left sides. A mollusk living in this 

 manner would naturally acquire a discoidal shell, and the coils 

 might not always be in the same plane. This is the case with 

 Planorbis, for we often find examples of the same species 

 which have the last one, two or three whorls either above or 

 below the median peripheral line of the whorls. We find 

 frequently that the apex and umbilicus are both more or less 

 perforating. This results in this manner : when the shell is 

 young the height is say one-eighth of an inch ; the next two 

 turns raise the general height to one quarter, and the original 

 apex is depressed below the level of the last whorl. This 

 continues until the adult form is reached, when we have a 

 species like amnion or trivolvis. I have known of cases where 

 the apex and umbilicus were so close together that the apex 

 finally became perforated, making a hole clear through the 

 shell. The animal is essentially sinistral, the generative, 

 respiratory, and excretory orifices being on the left side. The 

 following species have been critically examined, and I believe 

 the determinations can be depended upon: 



Planorbis trivolvis Say. I reduced this species to one 

 and a half whorls with results of 

 a positive nature. The apical whorls 

 were distinctly flat and regular, 

 planorbis tri- while the umbilical region was 

 Aduu' S form.' deeply perforated. There is planorbis trivolvis, 



. Say. Young speci- 



no question concerning the sinistrality of men of 2% whorls, 



,« . . showing sinistral 



thlS Species. character. 



Planorbis truncatus Miles, is decidedly sinistral, a fact 

 which is at once apparent without the trouble of dissection. 

 I am able to add two new localities to the original one at Sag- 

 inaw Bay, Michigan ; these are, Miller's, Indiana, and North 

 Branch of the Chicago River, near Bowmanville. 



Planorbis campantjlatus Say. This species is sinistral, 

 though at first sight it is taken for dextral. It is only after 

 the shell has been reduced to two or three whorls that the 

 sinistral character is seen. The umbilicus is not deep in this 

 species, as it is in trivolvis. 



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