138 The MoUuscan Family Plauorbidae 



In H. trivolvis fallax, a small race of trivolvis, a specimen was examined 

 in which the penial complex had a very large, pyriform preputiiim and a 

 small, cylindrical vergic sac (fig. 17). The penial gland of this specimen 

 was quite normal (fig. 18). 



The great variation in the form of the retractor muscle is well shown on 

 plate 29, especially in figs. 1, 2, 3, 5, 7, 8, 14, 15, 16, and 17. It varies from 

 a narrow band of muscle to a wide band with several branches. 



THE GENITALIA OF SPECIES ALLIED TO TRIVOLVIS 



Species East of the Rocky Mouxt.\ixs 



A closely related species, Helisoma chautauquense F. C. Baker (plate 29, 

 figs. 19-22), differs from the trivolvis complex in the wider, more pyriform 

 preputium, shorter and wider vergic sac, and especially the much shorter 

 penial gland duct (DC) which recalls the Seminolina group (fig. 22). The 

 penial gland, however, does not differ from that of trivolvis. In the imma- 

 ture stage of this species, the whole penial complex is wider, especially the 

 vergic sac, and the gland duct is very short, the whole male organ resem- 

 bling that of typical Helisoma (fig. 19). 



In a rather distinct species, Helisoma truncatum (]\Iiles), the genitalia 

 are similar in general form but show some minor peculiarities ( plate 26, fig. 

 13). The seminal vesicle (SV) is of greater diameter and less length, and 

 the ovisperm duct is of slightly greater length (4 mm.). The prostate 

 (PRS) is much shorter (1.7 mm.) and wider, and consists of fifteen 

 diverticula, which are on a short prostate duct placed at the right edge of 

 the group of diverticula (fig. 10). There are about eleven main diverticula, 

 several of which are liranciied at the end. The cross section of the prostate 

 of truncatian differs slightly from that of trivolvis. The duct of the sperma- 

 theca is somewhat longer than in trivolvis. The penial complex is wider 

 than in trivolvis and is similar to some of the races, lentum for example 

 (figs. 6, 7, 12). The penial gland (fig. 9) does not differ from that of 

 trivolvis. 



A larger species, Helisoma pilsbnji (F. C. Baker), living in Wisconsin, 

 Michigan, and the northern part of the United States and southern Canada, 

 is figured on plate 25. The genitalia (fig. 4) are similar in general form to 

 those of trivolvis, but there are some differences. The spermatheca (S) is 

 narrower and shorter (1.3 mm.) but the duct is long. The prostate is 

 shorter. The seminal vesicle is shorter and wider. The ovotestis diverticula 

 are very long. In the penial complex (fig. 5), the preputium is twice as long 

 as the vergic sac and both are wider than in tiivolvis. Variations in shape 

 and position of the vergic sac are shown in figures 1, 6, and 7. In the penial 

 gland, the cup is much shallower than in trivolvis (fig. 2). The genitalia 

 are somewhat shorter than in Helisoma corpulentum. 



A larger form than pilsbryi is known as H. pilsbryi infracarinatum 

 F. C. Baker which is common in southern Canada. While regarded as a 

 race of pilsbryi there are some features of the genitalia which indicate that 

 is a very distinct race (plate 25, fig. 13). The seminal vesicle (SV) and 

 ovisperm duct (SO) are similar to these organs in pilsbryi, but the prostate 

 (PRS) is twice as long (5.7 mm.) and much larger, comparatively. There 



