Subfamilies, Genera, and Subgenera — Recent and Fossil 187 



Geological Distribution. Pliocene to Recent fauna. Little is definitely 

 known concerning the geological ancestry of this genus. 



Remarks. Typical Menetus is a very distinct genus characterized by 

 noteworthy anatomical features, chief among which are the sac-like penial 

 gland with its short, narrow, internal duct and the epiphallus-like enlarge- 

 ment of the vas deferens as it enters the vergic sac. The prostate resembles 

 that of Planorbula which has the same structural arrangement of divertic- 

 ula. The ovotestis is also like that of Planorbula. The penial gland, how- 

 ever, is quite unlike either that of Planorbida or Promenetus. The duct is 

 also different . The radula and jaw are like these organs in Promenetus and 

 Planorbula. 



Menetus has been subordinated to several groups as a section or sub- 

 genus. Thiele phices it under Anisus ( 1931, p. 481 ) , Dall as a section under 

 subgenus Hippentis (1905, p. 82), Germain as a subgenus of Planorbis 

 (1923, p. 156). Von Martens (1899, p. 390) includes in it species now 

 referred to Tropicorbis. The Adams brothers indicated no type, though one 

 was definitelv proposed bv Dall in 1870 (p. 351). 



F. C. Baker (1928, p. 360) and Pilsbry (1934, p. 64) have shown 

 definitely that the group should rank as a genus. The anatomical figures 

 herein published support this conclusion. 



Subgenus MICROMENETUS F. C. Baker, New Subgenus 

 Genotype Planorbis dilatatus Gould 



Shell ("plate 79, figs. 13-15) . Very small, ultradextral, of few rapidly enlarg- 

 ing whorls ; right side flat or convex, left side with inner whorls submerged 

 by the body whorl; body whorl with a more or less well-developed carina, 

 usually placed just below the top of the right side; aperture large, some- 

 what dilated; lips thin. 



Animal. Foot short, wide, oval on base, bluntly rounded before and 

 behind; tentacles long and filiform, enlarged somewhat at base, attaclied 

 far back on the head; eyes on small swellings at inner base of tentacles; 

 color mottled brown and cream, whole back dark, two black lines extending 

 down middle of head; bottom and sides of foot yellowish; tentacles trans- 

 parent. Shape of foot varies during locomotion. The shell is carried at an 

 angle of thirty to forty-five degrees or it may lie flat on the body of the 

 animal. It is seldom carried erect as in Helisoma (plate 70, fig. 9). 



ANATOMICAL CHARACTERISTICS 

 PLATE 40 



GENITALIA. Male Organs (fig. 12). Seminal vesicle (SV) of M. dilata- 

 tu.s made up of a small number of relatively large vesicles placed on each 

 side of the ovisperm duct. Anterior to the seminal vesicle, the ovisperm duct 

 is bordered by a number of widely spaced vesicles for a distance three times 

 the length of the seminal vesicle. Sperm duct (SPD) of small diameter, 

 not much greater than that of the ovisperm duct. The combined free por- 

 tion and that part beneath the prostate measure about 1 mm. in length, 

 the sperm duct being a trifle longer than the prostate portion. Prostate 

 (PRS) with relatively few diverticula, eight or nine. In cross section, the 

 prostate shows the same multiplication of smaller diverticula on the under 

 side as is present in Menetus opercidaris. In an immature specimen of 



