OF NORTH AMERICA. 135 



1874. Limnaa HARTMAN and MICHENER, Conchologia Cestrica, p. 62. 



1876. Limnaa MEEK, U. S. Geol. Surv. Terr., final Report, IV, p. 532, 1876. 



1884. Lymnus TRYON, S. & S. Conch., Ill, p. 101, 1884. 



1902. Limn&a BAKER, Moll. Chi. Area, II, p. 259, 1902. 



1903. Limnus DYBOWSKI, Bull. Acad. Imp. Sci. St. Petersb., XVIII, p. 113, 1903. 

 1905. Lymncca DALL, L. & F. W. Moll. Alaska, pp. 59, 63, 1906. 



1908. Lymnaa BAKER, Science, n. s., XXVII, No. 703, p. 943. 



SHELL : Ovately-oblong or elongated, generally thin and brittle ; 

 spire more or less attenuated ; last whorl expanded ; peristome thin, 

 somewhat flaring, without internal varical thickening; aperture ovate 

 or oblong-ovate, sometimes rounded; axis gyrate, forming a sharp, 

 ascending columellar plait ; no true umbilicus, but there is occasionally 

 a very small chink; columellar callus closely appressed to the body of 

 the last whorl, forming a wide deposit; surface marked by numerous 

 fine impressed spiral lines and close-set longitudinal growth lines, form- 

 ing a finely decussated pattern; a horny periostracum or epidermis is 

 present. (PL XVIII, fig. 7.) 



ANIMAL : With a short, wide, rounded foot ; head with the usual 

 bilobed vela area; surface of body finely striated, presenting a smooth 

 appearance; tentacles, eyes and other characters as in the family. 



JAWS : Superior jaw much wider than high ; dorsal margin round- 

 ed, ventral margin with a central more or less acutely rounded pro- 

 jection; lateral jaws triangular, the lower angle produced into a long 

 tongue-like projection. (PI. VI, fig. A.) 



RADULA : . With unicuspid central tooth and bicuspid lateral teeth ; 

 marginal teeth serrated. (PI. VII, fig. A.) 



GENITAL SYSTEM : Penis-sac very large, penis very short (about 

 one-quarter the length of penis-sac) ; penis-sac retractors normally two 

 in number, very large; protractors of penis-sac very large, massive; 

 prostate forming a bulb-like enlargement at the entrance of the prostate 

 duct; the distal portion of the prostate is constricted behind the en- 

 larged portion and is somewhat fusiform in shape; receptaculum 

 seminis rounded, its duct of rather small diameter; lower portion of 

 oviduct very long; first accessory albuminiparous gland rounded. (PI. 

 X, fig. A.) 



TYPE : Helix stagnalis Linne. 



DISTRIBUTION : Holarctic region of America, Europe and Asia. 



The genus Lymncea is here restricted to include those species 

 having a bulb-shaped anterior termination of the prostate, differing 

 in this character from all known groups of Lymnaeidse. The penis-sac 

 retractors are normally two in number and are very massive. The 



