380 MOLLUSCA. 
PLANORBIS. 
Planorbis (Guettard, 1754), Geoffroy, 1767; O. Fr. Miller, Draparnaud, Lamarck, &c. 
Shell discoidal, each succeeding whorl in one and the same plane with those 
preceding, the shell thus appearing more or less hollowed in the centre (umbilicated) 
on both sides, but in most of the species not to the same amount. Plane of the 
aperture more or less oblique to the axis of the whorls. 
A pair of long, tapering feelers. 
Geographical distribution universal. 
For a fuller diagnosis of this remarkable genus, and as to whether the shell should 
be termed dextral or sinistral, the student is referred to Fischer and Crosse (Miss. 
Scient. Mex., Mollusca, ii. pp. 53 e¢ seg.). It is only necessary to explain here the 
terms which I have used in the comparative Table of the species and in the descriptions, 
and it may be noted that numerical measurements help better than words or figures to 
characterize the different forms of the shell. In order to avoid the terms ‘‘ upper face” 
and “lower face,” which are used in a different sense by different authors, I prefer to 
call these parts the right or left side of the shell, as the disc of the latter is usually 
borne more or less perpendicularly by the living animal: the right side of the shell is 
that which corresponds to the right side of the animal (the margin of the aperture 
being more extended on it than on the left side); in the European P. corneus and the 
species of the subgenus Helisoma it is more deeply excavated (umbilicated) than 
the left, but in the European P. contortus and P. nitidus the left side is more deeply 
excavated than the right. The right side is the upperside of most authors, especially 
of the older ones. ‘The height of the shell (alt.) is the greatest dimension of the shell 
in a plane parallel to the axis or across the whorls (this being usually situated at the 
aperture), but it must not be confounded with the breadth or amplitude of the 
aperture, which must be measured in the plane of the aperture, more or less oblique 
to the axis; the more oblique the aperture, the more its amplitude exceeds the height 
of the shell. The diameter of the aperture is its greatest dimension in the plane of 
the spiral line, 7. e. at a right angle to the axis; it can be measured directly in figures 
which represent the right side (upper face) of the shell, but not in those showing the 
profile or the left side of the shell. ‘The proportional relation of the last whorl is 
taken from the greatest diameter of the shell which is occupied by the last whorl at 
the aperture ; it must be measured in a line directly from the outer edge of the aperture 
to the centre of the shell, and is therefore not the actual diameter of the aperture, 
which extends in another direction: this proportional relation is required for the 
measurement. of the more or less rapid increase of the whorls. 
