REPORT 
ON THE 
CONTRIBUTIONS TO THE NATURAL HISTORY OF THE 
MOLLUSCA, DURING THE YEAR 1842. 
BY 
DR. F, H. TROSCHEL. 
B’Orbigny lias invented an instrument of general interest, 
for measuring the spiral angles of the convoluted shells of the 
Mollusca. He calls it a Helicometer. 
It consists of two arms, moveable by a joint, and to the one a semi- 
circle, divided into 180 degrees, is attached. If a shell is laid be- 
tween the two arms, the spiral angles can be immediately found out. 
D’Orbigny remarks, that Shells can thus be brought into three divi- 
sions : — 1. They have a spiral angle regular in its whole length : 2. The 
spiral angle is convex, or swollen in the middle : 3. It is concave. He 
points out the different ways of measurement applicable to these three 
forms. The increase of the spire is more or less rapid, and the oblique 
direction of the suture agrees with it. It is enough that a shell be placed 
in the Helicometer, with the mouth undermost, so that the one arm lies 
parallel to the axis or side of the spiral angle, whilst the other follows 
the suture. This measurement D’Orbigny calls the suture angle. He 
measures, besides, the length of the last whorl in relation to the whole 
length of the shell. (Institut. 1842, p. 52.) Unfortunately, the diffi- 
culty of exact measurement, and the irregularity of the spire in the 
same species of shell, will be an impediment to the introduction of this 
instrument into science. 
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