264 PLANORBINvE. 



elevatus, Say. orbiculus, Morel. 



exacutus, Say. pallidus, C. B. Adams. 



Gilberti, Dkr. Panamensis, Dkr. 



glaber, Jeff. parvus, Say. 



Hindsianus, Dkr. retusus, Morel. 



Isabel, Morel. Salleanus, Dkr. 



limosus, Dkr. thermalis, Beck. 



nautileus, Linn. troclioides, Bens. 

 nitidus, Mull. 



Genus SEGMENTINA, Fleming. 



Tentacles filiform. 



Shell orbicular, depressed, furnished internally with trans- 

 verse, testaceous partitions or teeth ; aperture transversely 

 oval, or circular. 



Syn. Hemithalamus, Leach. Segmentaria, Swains. 

 Discus, H aide man. 



Ex. S. armigera, Say,\>\. 84, fig. 4. Shell, S. lacustris, 

 Lightfoot, fig. 4, a. 



In S. lacustris the teeth-like processes of the shell are 

 three in number, and project so as to leave a tri-radiated 

 aperture for the passage of the animal ; the earlier ones 

 are not absorbed, and several of the partitions thus formed 

 are always present. In Planoriula, but one set of teeth, 

 which are more numerous than in the typical species of 

 the genus, exists in adult individuals, the septa formed at 

 earlier periods having become absorbed. 



Species of Segmentina. 



calatbus, Bens. Largillierti, Dkr. 



lacustris, Lightfoot. perforata, Gould. 



