No. 416.] IHE CONCHOMETER. 667 
paper. The object of having two scales is to avoid parallax 
errors in reading. The paper scale is protected from damp- 
ness (and consequent warping) by a sheet of transparent 
celluloid. 
A few words may be necessary to explain the working of 
the instrument. A shell of Purpura lapillus is lowered into 
the V, and the arms are adjusted to fit the apex. It is not a 
difficult matter to do this, and the adjustment may be made 
more accurately by having the plates polished to mirror-like 
brightness and using then the well-known method of determin- 
ing contact by reflection. The shell having been well fitted, 
the position of the point of the movable arm may be read on 
the before-mentioned arc, A, and the apical angle be thereby 
determined. A simple bob, B, is used to lower the shell into 
the V and to withdraw it. This is made of stiff copper wire, 
with a small ball of cotton wool dangling at the end of a piece 
of finer wire. The cotton wool is thrust into the aperture 
of the shell, and in this way the latter is held firmly, apex 
downwards. 
The total length of the shell and the distance from the apex 
to the aperture are both measured while the shell is in the. 
position just described and shown in the figure. It is important 
to note that the zero-point of the linear scale lies at the apex of 
the V, and that the apex of the introduced shell must neces- 
sarily be at that point. Hence the total length of the shell 
will be determined by observation of the part of the shell lying 
farthest from the zero of the scale, and when found it may be 
read directly by means of the concentric lines. In the same 
way the distance from the apex to the aperture may be read 
directly by noting where the aperture comes on the scale. 
Care should, of course, be taken always to present the ventral 
side for measurement. After some practice the measurements 
may be taken rapidly and accurately. 
Dow ACADEMY, FRANCONIA, N. H. 
