56 Records of the Indian Museum. EVOL. XVIIE, 
its real distribution. It certainly extends from Mesopotamia to 
the Kiangsu Province of China. 
The habits differ from those of G. convexiusculus in that the 
animal swims actively on the surface in the evening. This we 
recently observed at Chakradharpur in Chota Nagpur, and we 
have been able to confirm the observation in Calcutta. While 
floating shell-downwards like other species of the family with 
its foot applied to the surface film, it moves forward rapidly ina 
jerky manner by repeated strokes of its shell in the water. The 
sole of the foot adheres to the surface-film and the shell is raised 
almost to a horizontal position with its major axis parallel to 
but well below the film. It is then rapidly depressed, so that 
momentarily the axis forms almost a right angle with the surface. 
After this downward stroke it is rapidly raised again to a hori- 
zontal position, and the animal is propelled forwards a little ob- 
liquely. The manoeuvre is frequently repeated, each time with 
a jerk, leverage being provided by the friction between the sole of 
the foot and the surface-film. Doubtiess the flattened, carinate 
form of the shell is of advantage in its use as an oar, and pro- 
bably all species with this character behave in the same way. 
The bacterial “ velum’’, noted in the Burmese species G. velifer,' 
is commonly present in G. euphraticus also. 
Genus Segmentina, Fleming. 
1828. Segmentina, Fleming, Hist. Brit. Anim., p. 270. 
This genus consists like Gyraulus of small or minute, thin- 
shelled species, but the whorls are as a rule of a different form, 
convex above and flattened below and the shell is characterized 
by the production at intervals on the inner surface of the main 
whorl of curious opaque, white, transverse teeth or ridges of an 
enamel-like substance Correlated doubtless with the presence 
of these is the fact that the edge of the mantle is thickened. The 
radula differs from that of Gyraulus in having the teeth narrower 
and with smaller cusps and all the lateral multicuspid. The geni- 
talia are also of a different type in that the penis, though pro- 
duced into the penis-sheath, is directed into it from one side and 
is not provided with a horny stylet. 
Type-species. Planorbis nitidus, Miiller (Palaearctic). 
Segmentina calathus (Benson). 
1850. Planorbis calathus, Benson, Ann. Mag. Nat. Hist. (2) V, p. 348. 
1876. Planorbis calathus, Hanley and Theobald, op. cit., pl. xxix, figs. 
4-6. 
1918. Planorbis calathus, Annandale, Rec. Ind. Mus. XIV, p. 113. 
The shell is very much like that of the type-species of the 
genus (Planorbis nitidus, Miller), to which it apparently bears 
! Annandale, Rec. /nd. Mus., X1V, p. 112, pl. xi, figs. 7-11. 
