300 Note on a Race of Clmmlia TtaJa {Von Martens) 
columella is much shorter, the exactitude attainable by grinding is less ; but it is 
still sufficient for our purposes. 
A 
Fig. 1. The upper part of a section through Clausilia itala, showing the characters measured. 
In any section, points on the peripheral and columellar spirals are exposed, and 
by passing alternately from one side of the sensibly flat section to the otlier, we 
find points on either spiral separated by an angular distance of 180^. It is evident 
that the relative positions of two points, Pi and P., (Fig. 1) on the peripheral spiral, 
known to be 180° apart, are determined if we measure in the plane of the section 
the sides of the triangles AP^Gi and AP2C2. By proceeding in this way through 
the whole section, we might determine the law of growth of each side of the funda- 
mental triangle A PC, as it revolves round the columella, and so obtain data from 
which both peripheral and columellar spirals could be reconstructed. An attempt 
was made to do this ; but the columellar end of the septum between two successive 
whorls, which determines the length of PC, was found to be so indefinite that the 
measurement of this side of the triangle was abandoned. The " peripheral radius," 
AP, called P in the tables which follow, and the "columellar radius," AC, called 
C in the tables, were measured ; to obtain a measurement depending on the apical 
angle PAC, the perpendicular PM, from the peripheral spiral to the columella, 
was measured ; it was found that by means of the perpendicular cross-lines in the 
eye-piece of a reading microscope it was easy to determine this perpendicular 
(called in the tables p) with sufficient accuracy. 
The measurements were in all cases recorded to 0"01 mm., and care was taken 
to make them as accurate as possible; for purposes of tabulation, the more variable 
measures P and C were grouped so that the unit of tabulation was 0"05 mm., the 
less variable p was tabulated in units each equal to 0'02 mm. The individuals 
measured were 100 adults and 105 young, tlie whole number of measures made 
being well over 9000, though only a part are here discussed. 
