MOLLUSCS, 
oo- 
African form, has a reversed shell like Physa, but is distinguished by having a 
tooth or fold on the columellar margin of the aperture. In Ohilina the shell is 
dextral, like that of Limncea, 
but differs in being covered 
with a periostracum, and ex¬ 
hibiting reddish wavy colour 
markings. The columella is 
thickened and furnished with 
one or more folds or plaits. 
They are found only in clear 
running streams of South 
America. 
The curious pulmonate 
known as Amphibola some¬ 
what resembles a periwinkle 
in form. It lives between tide- 
marks in brackish or salt 
water, on mud-flats at the 
mouths of rivers in New 
Zealand, and is used as food 
by the natives. It is abundant 
in some places, and is a slug¬ 
gish creature, subsisting upon 
the vegetable matter contained 
in the mud, large quantities 
of which it passes through the 
alimentary canal. Professor 
Hutton says that it will live 
for a week or ten days in fresh 
water, and more than a fort¬ 
night in salt water, without 
being exposed to the air. The 
breathing-orifice is situated on 
the right side of the neck, and 
the radula shows some affinity to that of Pliysa. The shell is solid, globular, with 
a short spire and an oval aperture. The animal is furnished with a thin horny 
subspiral operculum. 
ramshorn snail [Planorbis corneus J. 
The Hind-Gilled Group, —Order Opisthobranchiata. 
The Opisthobranchs form the second of the three main divisions of the 
gastropods, and are all marine forms, having the sexes united in each individual, 
and breathing chiefly by gills or branchiae. This character at once separates them 
from the Pulmonata, and the different positions of the branchiae, and their herma¬ 
phrodite nature, serve to distinguish them from the Prosobranchia, the third and 
last main branch of the Gastropods. In the Opisthobranchs the branchial veins as 
