124 The Molluscan Family Planorhidae 



1899. HeUsoma Martens, Biol. Cent. Amer., Moll., p. 381. No type cited but includes 



Pierosoma. As subgenus. 

 1902. HeUsoma F. C. Baker, Moll. Chi. Area, II, p. 287. No type cited. As subgenus. 

 1905. HeUsoma Dall, Alaska Moll., pp. 81, 84. Type Planorbis bicarinatus Sowb. 



As subgenus. 

 1912. HeUsoma Hannibal, Proc. Mai. Soc. London, X, p. 157. Type Planorbis 



antrosus Conrad. As genus. 

 1918. HeUsoma Walker, Miscel. Pub., Mus. Zool., Univ. Mich., No. 6, p. 11. Type 



Plaiiorbis bicarinatus Say. As subgenus. 

 1921. HeUsoma Germain, Rec. Ind. Mus., XXI, pp. 6, 64. Type Planorbis carinatus 



Say. As subgenus. 



1923. HeUsoma Wenz, Fossil. Cat., Pars 22, p. 1522. Genotype HeUsoma bicarinata 

 (Sowb.). As genus. 



1926. HeUsoma F. C. Baker. Trans. Wis. Acad. Arts and Sci., XXII, p. 201. Type 

 Planorbi^s bicarinatus Sowb. As genus. 



1928. HeUsoma F. C. Baker, Fresh-water Moll. Wis., I, p. 311. Type Planorbis 



bicarinatus Sowb. As genus. 



1929. HeUsoma Chamberlin and Jones, Bull. Univ. Utah, Biol. Ser., No. 1, p. 145. 



Genotype Planorbis bicarinatus Sowb. As genus. 



1930. HeUsoma H. B. Baker, Occ. Papers, Mus. Zool., Univ. Mich., No. 210, p. 43. 



Planorbis bicarinatus Say. As genus. 



1931. HeUsoma F. C. Baker, Proc. Zool. Soc. London, p. 584. Type Planorbis bi- 



carinatus Sowb. As genus. 



1931. HeUsoma Thiele, Handbuch, Teil 2, p. 479. Type Planorbis bicarinatus Say. 

 As section of Planorbis. 



1934. HeUsoma Pilsbry, Proc. Acad. Nat. Sci. Phil., 86, p. 47. Type Planorbis bica- 

 rinatus Say. As genus. 



Subgenus HELISOMA SS. 

 Type Planorbis anceps Menke 



Shell (plate 80, figs. 1-6). Large, sinistral, of comparatively few 

 whorls which rapidly increase in size, carinated; spire and umbilicus 

 funicular; aperture expanded, outer lip thickened. 



Animal. Yellowish or brownish, flecked with whitish; there are s]5ots 

 of color near the eyes and between the tentacles, and the mantle is spotted 

 with dark mottling, especially over the kidney. The tentacles are long and 

 filiform and the velar area is notably developed. The shell is carried almost 

 perpendicularlv. 



ANATOMICAL CHARACTERISTICS 



PLATE 23. HeUsoma anceps (Menke) 



GENITALIA. Male Organs (fig. 11). Seminal vesicle (SV) short (about 

 1 mm. long) and widely rounded, composed of many small follicles. An- 

 terior to the seminal vesicle, for the space of 2.5 mm., the ovisperm duct 

 is bordered by many small follicles which may represent a portion of the 

 seminal vesicle (SO). The sperm duct (SPD) is about 6 mm. long and is 

 of wide diameter, somewhat sausage-shaped, the tube about 0.3 mm. in 

 diametei'. The prostate (PRS) is ovate in shape, about 2.5 mm. long, 1 mm. 

 wide and less than 1 mm. in height. It is composed of numerous diver- 

 ticula forming a pavement-like pattern when viewed externally. In section 

 (fig. 9) the prostate is fan-shaped, composed of six to eight diverticula 

 each of which is branched two to four times toward the distal end. The 

 prostate is placed on a short prostate duct which unites with the sperm 

 duct to form the vas deferens (plate 22, fig. 12) . The vas deferens (VD) 



