THE CONCHOMETER. 
HAROLD S. CONANT. 
In connection with a recent study of variation in Purpura 
lapillus, it seemed necessary to measure the apical angle of the 
shell, among other features, and since no apparatus adapted to 
this purpose was known, an instrument, which I have called a 
conchometer, was constructed. 
About 4000 measurements have been taken by this means ; 
and so well did they correspond with theoretical considera- 
tions that the writer believes the value of the instrument to 
have been proven. With the possibility that those who are 
using the statistical method for studying variation may find it 
suited to their purpose, a description is here given. 
It consists of two parts : (1) a device for measuring angles 
at the ends of the shell, and (2) a device for measuring the 
long axis of the shell and the distance from the apex of the 
shell to the aperture. The first consists of an ordinary draw- 
ing compass, of which one arm, ~, is firmly attached to a 
board, and the other, M, may be moved over a graduated arc, 
4, which is also attached to the board. The arc reads to half- 
degrees and has its zero-point at the fixed arm. Attached to 
each arm of the compass is a plate of metal some 2 inches 
long and ł of an inch wide, standing vertically to the board, so 
that only the edge is visible in the figure. The two plates are 
so placed on the arms that they just touch each other at the pivot 
of the compass. When the board is set in position, vertically, 
the plates form a V whose angle may be made larger or smaller 
by the adjustment of the movable arm by means of a suitable 
handle, Æ. It is with this part of the apparatus that the apical 
angle of the shell may be measured, as described later. 
The second part of the apparatus, by means of which linear 
measurements are taken, consists of two scales graduated to 
millimeters by means of concentric lines. One is etched upon 
665 
