156 Records of the Indian Museum. [Vor,. XT, 
end; this free anterior end of flap gives attachment to the 
radula. Inside the muscular sheath and lying over the flap is 
a bilobed hollow fibrous cushion with a thick and hard wall. 
This cushion is free at its round anterior end and ventral aspect, 
but is attached to the muscular sheath behind and laterally 
towards the dorsal aspect. Ventrally between the two lobes is a 
deep transverse fissure into which the anterior end of the ventral 
flap fits. The dorsal surface of the cushion is convex in the 
middle line, but concave at the sides, where it is continuous 
with the inner surface of the muscular sheath. A little anterior 
to the middle of its length is a transversely placed crescentic 
aperture leading into the radula sac proper. The vadula is 
attached to the anterior end of the ventral flap and passes 
over the anterior rounded end of the cushion to its dorsal 
surface over which it is traced backwards into the radula sac 
proper. The portion of the radula lying over the dorsal surface 
of the cushion extends laterally on the concave lateral portions 
of the surface and the inner surface of the muscular sheath, 
so that in a longitudinal section a little to one side of the middle 
line we get two sections of the radula—one lying on the dorsal 
surface of the sac, and another beneath the inner surface of the 
inner muscular sheath at a higher level than the first. 
Minute structure of the radula and the radula sac proper.— 
The vadula consists of the following layers :— 
(1) A thin fibrous membrane lined beneath by a_ single 
layer of pavement epithelium. The membrane con- 
sists of white fibres alternating with single rows of 
connective tissue corpuscles. 
(2) A single layer of cubical epithelium over the fibrous 
layer. 
(3) A thick homogeneous corneous layer with fine longi- 
tudinal striation. To this are attached the bases of 
the teeth which are all unicuspid and are arranged in 
V-shaped rows. 
The vadula sac lies in the middle of the bilobed cushion in 
its dorsal aspect. The cyescentic aperture (mentioned above) 
leads into the narrow cavity which is directed downwards and 
backwards, and which ends blindly after curving a little back- 
wards and outwards. At the sides the cavity extends downwards 
and outwards and then upwards and inwards again for a short 
distance, where it ends blindly abutting on the wall of the hollow 
mass on the dorsal aspect. In a longitudinal section of the sac, a 
little to the side of the middle line, we get asort of horse-shoe-shaped 
appearance as the cavity extends for some distance on the anterior 
aspect where the two portions become continuous. Ina transverse 
section through the middle of the sac we get a reniform outline 
with the middle third of the convex side absent. 
The sac is surrounded by a sheath of connective tissue. 
The ventral and the outer walls of the sac are thin; the lower 
