74 The Molluscan Family Planorhidae 



striatus (fig. 2) the kidney is much shorter and wider (less than 2 mm. 

 long). In vermicularis it is short and wide as is also the case in similaris. 

 Cross sections through the kidney of the four species show a similarity of 

 conditions (fig. 5 parvus; fig. 6 circumstriatus ; fig. 7 vermicularis; fig. 9 

 similaris). The kidneys in Gyraulus and Torquis differ little in structure. 



Digestive System. The digestive tract shows division into crop, gizzard, 

 and pyloris and there is a large blind sac. The intestine does not form a 

 posterior loop, as in typical Gyraulus, but crosses over the oesophagus and 

 extends forward as in the genus Planorbis. The buccal sac is like this organ 

 in typical Gyraulus. 



The jaw (plate 50, fig. 16, parvus) is horseshoe-shaped with many 

 small plates as in Gyraulus typical. There are approximately thirty-six to 

 forty plates on the jaw of parvus and Gyraulus vermicularis has about 

 forty-seven plates (fig. 19). Gyraulus circumstriatus (fig. 22) has a small, 

 narrow jaw with about thirty-seven plates. The jaws of Gyraulus (figs. 

 17-18) are not as typically horseshoe-shaped as in the subgenus Torquis. 



The radula formula of Gyraulus parvus (Say) is 14-1-14 with 105 rows 

 of teeth (plate 69, fig. 3). The center tooth is bicuspid, the cusps not 

 reaching the lower margin of the base of attachment. There are 1-6 lateral 

 teeth, tricuspid, with spade-shaped cusps. Intermediate teeth (7-8) with 

 the ectocone split into two small cusps. The 9th tooth is also transitional 

 with the ectocone split into three small cusps. Typical marginal teeth 

 (10-13) have the entocone split into two small cusps, the mesocone large 

 and spade-shaped, and the ectocone split into three small cusps. 



In Gyrauliis vermicularis (Gould) (fig. 5) the formula is 15-1-15, with 

 1-6 laterals, 7-10 intermediates and 11-15 marginals. There are 120-125 

 rows of teeth. In Gyraulus similaris (F. C. Baker) (fig. 4) the formula is 

 22-1-22 with 1-7 laterals, 8-10 intermediates, and 11-22 marginals. There 

 are 130 rows of teeth. Gyraulus circumstriatus (Tryon) has the formula 

 14-1-14 with the same number and position of teeth as in parvus. The 

 radula teeth of the subgenus Torquis do not differ in general character- 

 istics from those of typical Gyraulus. Compare figs. 1, 2 with figs. 3, 4, 5 

 on plate 69. 



For the above anatomical data the following specimens were studied : 



Gyraulus parvus (Say), 14 specimens from Winnebago Lake, Wisconsin, Henry Co., 

 Illinois, and Meach Lake, Canada, collected by F. C. Baker and A. LaRocque. 



Gyraulus circumstriatus (Tryon), 17 specimens from Wainwright Park, Alberta, col- 

 lected by Dr. Swales, and North Star Lake, Wisconsin, collected by F. C. Baker. 



Gyraulus vermicularis (Gould), 10 specimens from Mt. Lake, San Francisco, California, 

 and Hat Creek, Bishop, California, both lots collected by Dr. G. D. Hanna. 



Gyraulus .siyiiilaris (F. C. Baker), 17 specimens from Smartweed Lake, Toland, Colo- 

 rado, collected by Dr. Frank Smith. 



Geographical Distribution. The subgenus Torquis is found throughout 

 North America from Mexico northward. It is also apparently represented 

 by Gyraulus santacruzensis Germain from the Island of St. Croix, West 

 Indies. Gyraulus laevis Alder, a common species of Europe, also belongs to 

 this subgenus. Some of the other species listed under the distribution of 

 typical Gyraulus may belong to Torquis, but not enough material is avail- 

 able for study to confirm this suggestion. 



