HANNIBAL: CALIFORNIAN FRESHWATER MOLLUSCA. 159 



P. Samsoni, Ancey, and Alahamensis, Pils., are two of the better 

 known syuonyras, which have, however, not been used for the species 

 in the present district. 



' Plaiiorbis {SpirorbisY lunatus, Conrad, 1871. 



Oligocene : John Day Series, Oregon. 



This is not a Planorbis, as supposed by Conrad, but a land snail 

 belonging to the genus Ammonitella of J. G. Cooper. Stearns, 

 apparently unaware of Conrad's name, redesci'ibed the species in 

 1900 as Ammonitella Yatesi precursor. Ammonitella liinata is, however, 

 specifically distinct from A. Yatesi, a recent species from the Sierra 

 Nevada Mountains, California. 



Genus Helisoma, Swainson. 

 Helisoma, Swainson, 1840 [Planorbis bicarinafus, Sowb. = P. antrosus, 

 Conr.) ; TapMus, H. & A. Adams, 1855 (P. andecoU, d'Orb.). 

 Type, Planorbis antrosus, Conrad. 



Sub-genus Planokbella, Haldeman. 

 Helix (sp.), Liune, 1758 {II. cornea, L.); Planorbis (sp.), Miiller, 

 1774 {P. purpura, Miill. = H. cornea, L.); Planorbella, Haldeman, 

 1842 {P. campanulatiis, Say); Planorbina, Haldeman, 1842 

 {P. olivaceus, Spix, cited by Hall, 1905); Coretiis, ' Adanson,' 

 Gray, 1847 (II. cornea, L.); Menetus, H. & A. Adams, 1855 

 (for ^ Anisus, Beck, not Fitz.', hence P. olivaceus, Spix); Adula, 

 H. Adams, 1861 (P. multivulvis, Case = P. campanulatus, Say, 

 syntonic form), not Adula, H. & A. Adams, 1851; Ancceus, 

 H. Adams, 1869 (for Adula, H. Adams, hence same type), not 

 Ancceus, Frauvel, 1863 ; Pierosoma, Dall, 1905 (P. trivohis, Say). 

 Type, Planorbis campanulatus. Say. 



Sub-genus Perrinilla, n. sub-gen. 

 Type, Helisoma CordiUerana, n.sp. 



Named in honour of Dr. James Perrin Smith, to whom the writer is 

 under obligations for frequent advice, particularly pertaining to the 

 theoretical problems, during the preparation of this paper. 



Sub-genus Planorbella. Shell large, averaging 20 mm. in diameter, 

 whorls moderately deep, sinistral, the spire planulate, teiuiing to 

 become excavated as the adult condition is reached, its margin 

 sub-carinate, umbilicus narrow and deep, broadening rapidly during 

 later development, margin subangular, apertui'e retracted above, 

 expanded in adult ; habitat lakes and quiet streams. 



Sub-genus Helisoma, s.s. Shell similar to Planorbella, but distinctly 

 ultra-sinistral, the spire-pit and umbilicus funicular, and nearly equal 

 in depth, each margined by a decided periplieral carina, aperture 

 expanded in adult; Planorbella stage passed very early during 

 adolescence ; habitat similar to Planorbella. 



Sub-genus Perrinilla. Shell similar to Planorhella, except that 

 it is totally ultra-sinistral, spire-pit deep and narrow, umbilicus but 

 slightly excavated, superior and inferior peripheries subangular ; 

 Helisoma stage passed very early during adolescence, Planorbella 



VOL. X. — JUNE, 1912. 11 



