54 The Molluscan Family Planorbidae 



short V-shaped lobe to the right of the pseudobranch. It is large and can 

 be greatly expanded. 



The kidney (plate 46, fig. 9) is very long and narrow (10 mm. long, 

 1 mm. wide), the glandular portion occupying only about one-eighth of the 

 length. The ureter is short (1 mm. long), narrowly tubular and turns 

 sharply to the left into the mantle cavity. A cross section of the kidney at 

 about midway of its length shows the lumen to be flattened and widened 

 with a vein at each end (fig. 10). There is no ridge on the lower side. The 

 pericardium is long-oval in shape, 2.5 mm. long and 1.2 mm. wide. 



Digestive System. The stomach (plate 48, figs. 6, 7) is elongated and 

 pyriform in sliape, the blind sac about as long as the stomach. The intes- 

 tine runs directly forward and does not make a backward loop as in 

 Helisoma and some other groups. The stomach does not show distinct 

 divisions into crop and gizzard. The buccal sac is elongated and shortly 

 pyriform in shape, the fore part near the mouth somewhat tubular. The 

 radula sac is distinct and rounded. The salivary glands are much elongated, 

 cylindrical, and joined behind. 



The jaw (plate 50, fig. 21) is horseshoe-shaped and is composed of 

 many small plates, about thirty on the upper or horizontal part, the lower 

 or cutting edges V-shaped. 



The radula (plate 67, fig. 7) formula is 24-1-24, 25-1-25, or 26-1-26 

 and the number of rows of teeth varied as follows: 175, 176, 205, and 209. 

 The central tooth is bicuspid with a wide base of attachment, tlie cusps 

 long and spade-shaped; lateral teeth (1-10) tricuspid, the cusps large and 

 spade-shaped, the mesocone largest and longest, the reflection reaching 

 nearly to the lower edge of the base of attachment. Intermediate teeth 

 (11-13) with the reflection reaching only about half way to the lower edge 

 of the base of attachment, with four cusps, the ectocone split into two 

 cusps. Marginal teeth (14-20) very narrow with the reflection placed high 

 up on the tooth, with four cusps which become subequal toward the outer 

 margin of the radula membrane. The outer marginals are very narrow, the 

 width being about one-fourth of the heiglit and there are four short, sub- 

 eciual cusps. In one membrane some of the marginal teeth (20) had the 

 entocone split into two sharp cusps. This was rare and occurred only in 

 atyjiical rows. 



The central teeth varied somewhat in width in the same membrane. For 

 example, in rows 1 to 156 the upper part of the central tooth measured 6 

 microns and the lower part of the base of attachment measured 12 microns 

 in width. In rows 157 to 176 these measurements were 5 and 12 microns 

 respectively. In most membranes the central teeth were very uniform in 

 width. 



For the above anatomical data eight specimens were examined, col- 

 lected by Mr. A. Jankowski from a pond in Swidry-Mate, 25 km. south- 

 east of Warsaw, Poland. No parasitic worms (cercariae) were observed in 

 any of the specimens examined. 



Geographical Distribution. The genus Planorbis, as herein understood, 

 is distributed throughout Europe from Sweden southward to the Medi- 

 terranean and from Great Britain eastward to western Russia. It is also 

 found in northern Asia (Siberia), Asia Minor, and northern Africa. 



Species Considered as Valid. Although many species now assigned to 

 other genera were originally placed in the genus Planorbis there are now 



