514 MOLLUSC A 



SUBORDER 2. STYLOMMATOPHORA.* 



Land snails with 2 pairs of retractile tentacles, the hinder and larger 

 pair bearing a pair of eyes at their tips (Fig. 771) ; shell absent or con- 

 cealed in the slugs, and in the other forms usually with 4 to 6 whorls 

 which form a more or less conical spire (Fig. 772) ; respiratory pore on 

 the right side of the body (Fig. 771, 5), with the anal and excretory pores 

 near it; either 2 (Ditremata) genital pores or 1 (Monotremata) ; in the 

 latter case, which characterizes all the American families except the 

 Onchidiidae, the pore is near the base of the right hinder tentacle (Fig. 

 771, 1) ; ureter passes either directly forward from the kidney towards 

 the anterior margin of the lung (Orthurethra), or is reflexed, passing 

 first to the posterior end of the lung cavity (Sigmurethra) : about 20 

 families, with about 15,000 species. The American forms fall into several 

 distinct groups. These are (1) the indigenous American snails, including 

 almost all those species occurring east of the Sierra Nevada and Cascade 

 mountains; (2) the boreal species, certain small northern forms of cir- 

 cumpolar distribution; (3) the Pacific slope species, which have migrated 

 there from Asia by way of Alaska; (4) the European species, introduced 

 by human agency; and (5) the southern species, which have entered the 

 country from Central America and the West Indies. 



Key to the families of Stylommatophora here described: 



Oi Shell present. 



Z>! Shell with a high spire, being conical or cylindrical, not heliciform 

 (except Strobilops) . 



G! Shell more or less cylindrical, pupiform (Fig. 788) 2. PUPILLIDAE 



c a Shell conical ; with an acute apex. 

 d { Jaw present. 



e^ Spire very small, composed of 2 or 3 whorls 5. SUCCINEIDAE 



e, Spire high, composed of 5 or more whorls. 



A Jaw smooth ; shell glossy 3. COCHLICOPIDAE 



/ a Jaw ribbed ; southern animals 8. BULIMULIDAE 



d t Jaw absent 10. OLEACINIDAE 



5 a Shell heliciform (Fig. 772) or depressed (except 8 phy radium) . 

 G! Shell with a wide lip. 



di Shell minute, less than 3 mm. in diameter 4. VALLONIIDAE 



d 2 Shell not minute, more than 6 mm. in diameter 6. HELICIDAE 



c t Shell with a thin lip. 



</! Jaw without a median projection 7. ENDODONTIDAE 



d 3 Jaw with a median projection. 



ej. Teeth of radula thorn-shaped, in V-shaped rows 9. CIRCINARIIDAE 



e 3 Teeth both thorn-shaped and bicuspid and in straight rows. . 11. ZONITIDAE 



* See "Nomenclature and Check List of North American Land Shells," by H. A. 

 Pilsbry, Proc. Acad. Nat. Sci., Phila., 1889, p. 191. "Preliminary Outline of a New 

 Classification of the Helices," by H. A. Pilsbry, same, 1892, p. 387. "Guide to the 

 Study of the Helices/' by H. A. Pilsbry, Vol. 9, Ser. 2, Man. of Conchology, 1894. 

 "A Classified Catalogue of Am. Land Shells with Localities," by H. A. Pilsbry, 

 Nautilus, Vol. 11, pp. 45 et seq., 1897-98. "On the Zool. Position of Partula and 



