Subfamilies, Genera, and Subgenera — Recent and Fossil 149 



from Cuba, Jamaica, Haiti, and other islands. The metropohs of the group 

 is in North America, principally in southern Canada and northern United 

 States, where the largest number of species and races is found. Southward 

 the number of species rapidly decreases, only two species being known from 

 South America and four from the West Indies. These appear to be species 

 which have migrated southward in past geological time, possibly by the 

 aid of migrating birds. 



In the recent fauna, no member of the Helisoma group is found outside 

 of North and South America. In Pleistocene deposits, however, a form of 

 Helisoma allied to trivolvis has been reported by Lindholm (1933) from 

 the Kolyma River in northeast Siberia beyond the Arctic Circle. It would 

 appear that this planorbid is not referable to trivolvis but to some form 

 like horni or subcrenatum. The latter species is common in northwestern 

 British America and also occurs in Alaska. The Siberian form should 

 probably be allied with subcrenatum. Lindholm called it Helisoma trivolvis 

 koly7nense but it might be better to designate it simply as Helisoma koly- 

 mense Lindholm until more is known about its affinities. As suggested by 

 Pilsbry (1933, p. 137), it might represent a migration by the way of the 

 old land bridge between Alaska and Siberia, used by both man and other 

 mammals during Pliocene and Pleistocene times. 



Species Considered as Valid. The large number of species attributable 

 to the genus Helisoma, subgenus Pierosoma, are listed below. The names 

 Helisoma (Pierosoma) are understood to be prefixed before each specific 

 name in the list. 



ammon (Gould) plexatmn (Ingersoll) 



binncyi (Tryon) salvini (Clessin) 



calodermum (Pilsbry) subcrenatum subcrenatum (Carpenter) 



caribaeum caribaeum (d'Orbigny) subcrenatum disjectum (Cooper) 



caribaeum guatemalense (Clessin) tenue tenue (Philippi) 



chnutauquensc F. C. Baker tenue boucardi (Fischer and Crosse) 



contrcrasi (Pilsbry) tenue calij orniense F. C. Baker 



corpulentum corpidentum (Say) tenue chapalense (Pilsbry) 



corpulentum vermilionense F. C. Baker tenue exaggeratum (Martens) 



costaricense (Preston) tenue pertenue F. C. Baker 



equatorium (Cousin) (= applanatus Martens preoccupied) 



eyerda??ii Clench and Aguayo tenue sinuosum (Bonnet) 



jovenle (Menke) {^ affine C. B. Adams) tenue strebeUanum (Fisch. and Crosse) 



horni (Tryon) traskii (Lea) 



kolymense Lindholm trivolvis trivolvis (Say) 



viagnificum (Pilsbry) trivolvis fallax (Haldeman) 



mullicostatum multicostatum F. C. Baker trivolvis lentum (Say) 



multicostatum whiteavesi F. C. Baker trivolvis macrostomiim (Whiteaves) 



occidentale occidentale (Cooper) trivolvis turgiduni (Jeffreys) 



occidentale depressum F. C. Baker (= intertextum (Sowb.) ) 



oregonense (Tryon) truncatum (Miles) 



peruvianum (Bred.) tumens (Carpenter) 



pilsbryi pilsbryi (F. C. Baker) winslowi (F. C. Baker) 



pilsbryi injracarinatum F. C. Baker ivyldii (Tristram) 



Remarks. Pierosoma is a very distinct group of Helisoma, distinguished 

 from the subgenera Helisoma and Seminolina by peculiarities of genitalia 

 and radula. The duct of the penial gland is always longer in adult animals 

 than in the other groups mentioned. Some of the special anatomical features 

 have been described and figured in the preceding pages. The shell is large 

 and distinctly sinistral, especially in immature specimens. Pierosoma is the 

 dominant planorbid group in North America north of ]\Iexico. 



