438 MISS LETTICE DIGBY ON THE STRUCTURE OF 
External Features.—The general features of the mantle-cavity 
(Pl. 38. fig. 9), the character of the tentacles, the position of the 
eyes and of the apertures, are all very like those already deseribed 
in Chytra, the most apparent differences being the smaller size 
of Limnotrochus Thomsoni, the flatter shape of the visceral hump, 
and the pigmented band down the centre of the snout and on 
the inner sides of the tentacles. 
Nervous System.—The nervous system is zygoneurous on the 
right (Pl. 38. figs. 5,6). The ganglia and nerves are protected 
by a closely adhering connective tissue which contains skeletal 
elements. The cerebral ganglia (¢.c.) are separated by a distinct 
cerebral commissure, and are situated decidedly further back 
than in Chytra. The right pleural ganglion (4.p.') gives off a 
fairly long supra-intestinal cord, which connects the right pleural 
ganglion with the supra-intestinal ganglion. A fine nerve comes 
off from this cord just before it unites with the supra-intestinal 
ganglion. This nerve appears to join with another arising from 
the left pleural ganglion, but I was not able, actually, to see the 
connection. The supra-intestinal ganglion (g.sp.) gives off the 
left visceral nerve. The sub-intestinal cord springs from the 
left pleural ganglion, and soon passes into the sub-intestinal 
ganglion (.s2.) which, in the usual way, gives off the right pallial 
and right visceral nerves. A very short nerve (z) connects the 
sub-intestinal ganglion directly with the right pleural. Viewed 
from the side (fig. 6), the relative position of the cerebral and 
pleural ganglia is rather curious, the pleural ganglia being 
posterior and ventral to the cerebrals and nearly fused with the 
pedal ganglia, from which they are separated by a very short 
pleuro-pedal commissure (c.p.pe.'), the cerebro-pedal commissure 
(c.c.pe’.) beg longer. Each pedal ganglion gives off two nerves 
to the foot, but I was not able to see the nerve leading to the 
otocysts. The otocysts contain many rectangular otoliths 
(Pl. 38. fig. 7), 
Radula.—The radula (PI. 38. fig. 4) is distinctive, its most 
striking feature being the blunt protuberance on the underside of 
each of the two lateral teeth. The admedian tooth is large, and has 
a broad upper portion, bluntly serrated along its anterior face, 
which overhangs a lower portion, whose sharply serrated eee) is 
turned towards the inedian tooth. 
Viscera.—The mouth leads into a short buccal mass, A pair 
of salivary glands, diminutive sacculated organs, open into the 
