160 PROCEEDINGS OF THE MALACOLOGICAL SOCIKTY. 



stugo not distinguished on iiecount of preservation ; habitat apparently 

 similar to Planorbella. 



Nearly every well-established recent species of this genus has 

 apparently served as the type of its own peculiar sub-genus. The 

 present arrangement is not liable to offer particular difficulties to 

 American concliologists, but the Planorbella group is commonly known 

 in Europe under the name of Cortetis. This was originally used by 

 Adauson (as Coretus) in a somewhat different sense, but on its 

 introduction into Linnean nomenclature it was transferred to the 

 present group. Since this did not take place until after the appearance 

 of Planorbella it must be suppressed, a procedure that the writer is 

 not sorry for on historical grounds. 



ITelisoma appears to be a Mesozoic genus, which reached its 

 culmination in the older Tertiary or earlier, and is represented in 

 the living state chiefly by species belonging to the primitive group. 

 Perrinilla, which would doubtless be termed by Grabau a ' second 

 round-whorled stage ', appears to be an instance of over-specialization 

 resulting in extinction. 



Helisoma (Planorbella) trivolvis (Say). 



Planorhis trivolvis, Say, 1817; P. tumidus, Pfeiffer, 1839 (syntonic 



form); P. ^ corpulentus, Say', Haldeman, 1814 (syntonic form) ; 



P. ^ glabratuH, Say ', Haldeman, 1844 {parlim); P. amnion, Gould, 



1855 (syntonic form); P. suhcrenatus, Carpenter, 1856 (senile); 



P. 7'raski, Lea, 1856 (syntonic form) ; P. tumens, Carpenter, 



1857 (syntonic form) ; P. truncalus, Miles, 1861 (syntonic form); 



P. Horni, Tryon, 1865 (syntonic form) ; P. Oregonensis, Tryon, 



1865 (syntonic form) ; P. Binneyi, Tryon, 1868 (syntonic form); 



Helisoma ' tenuis, Phil.', Carlton, 1870 (syntonic form) ; P. occi- 



denfalis, J. G. Cooper, 1870 (syntonic form) ; JI. plcxata, lugersoll, 



1874 (syntonic form); P. {subcrenatus, var. ?) disjectus, J. G. 



Cooper, 1890 (syntonic form); P. ' lentus, Say', Stearns, 1893 



(syntonic form) ; P. ' vermicularis, Gould', Arnold, 1903. 



Shell large, coarsely striate, whorls deep, not numerous, superior 



margin sub-prominently carinate, forming an evenly concave spire-pit, 



aperture somewhat expanded in adult ; habitat lakes and sluggish 



streams. 



Entire Nearctic Region. Mexican Province. 



Quaternary : Loess of eastern States ; San Pedro Formation 

 (specimens washed into marine terraces), alluvial deposits of San 

 Joaquin Valley, and Le Conte Lake beds, California ; Lahontan Lake 

 beds, Nevada ; Bonneville Lake beds, Utah ; post-Glacial deposits of 

 Vancouver Island. Pliocene: Santa Clara and Cache Lake beds, 

 California. 



Helisoma antrosa (Conrad). 

 Planorbis bicarinatus, Say, 1817, not of Lamarck, 1804; Helix 

 angulata, Rackett, 1821, not of Burrow, 1815 ; P. antrosus, 

 Conrad, 1834 ; P. elongatus, Conrad, 1835 ; P. bicarinatus, 

 Sowerby, 1840, not of Lamarck, 1804; P. angistoma, Haldeman, 

 1844; P. lautus, H. Adams, 1861. 



