ANNUAL GROWTH OP FRESH-WATER MUSSELS 
715 
the degree of the disturbance. The duplication of layers of periostracum and pris- 
matic substance, but particularly the former, gives the appearance of a dark band 
on the shell, which is the so-called growth ring. These authors suggest that this 
ring might be better termed duplication ring, or interruption ring. They also found 
that the annual rings which are associated with the cessation of growth in the winter 
season are actually formed by repeated startings and stoppings of growth, in both 
the late fall and early spring, due to the passing warm and cold spells, so that the 
annual rings thus produced are usually broad, compound rings, quite different from 
the single narrower interruption rings resulting from more temporary disturbances 
of growth. Grier (1922) used the rings of certain Ohio species to check the age of his 
specimens in a study of relative rates of growth of various lake species. 
It is evident then that the various writers agree that the rings in the shells of 
fresh-water mussels are caused by cessation in growth and that the rings formed 
during the winter period are, in the main, heavier and better marked. It remains, 
therefore, in the application of these rings to a study of annual growth, to devise a 
method by means of which the annual rings — that is, those rings formed by the cessa- 
tion of the major period of annual growth — may be differentiated from the lesser 
lines produced by temporary cessation of growth due to temporary unfavorable 
conditions. 
In the case of many species of fresh-water mussels, particularly many members 
of the Lampsilinae and other comparatively thin-shelled species, the growth inter- 
ruption rings developed from all causes are partially apparent to the unaided eye. 
Illumination of the shells, obtained by placing an incandescent bulb immediately 
behind one valve of the shell, was used with considerable success in the earlier work 
on the light-colored yellow sand shells and Lake Pepin muckets. Such illumination 
brings out the full extent of each ring and was of great aid in separating the more 
conspicuous annual rings. 
An improvement was made later by using monochromatic yellow light, in the 
place of ordinary light, for the illumination of the shells. The single valve was placed 
on a plate of monochromatic yellow glass (Corning glass, No. G38-H) through which 
the light from an incandescent bulb was passed. This yellow light was found to be 
particularly effective in bringing out the rings in the thinner-shelled species. Some 
of the shells were placed in front of a powerful ultra-violet light. The ultra-violet 
rays caused the calcium in the shell to floresce with a yellow-green light which caused 
the thicker annual rings to stand out sharply in contrast. The use of ultra-violet rays 
was found quite effective in differentiating the annual rings in even thick shells of 
the Quadrula group (results to be given in another paper), but for the shells used in 
the present studies, the monochromatic yellow light was found amply satisfactory. 
In addition to the measurement of length — that is, the greatest antero-posterior 
distance measured as a chord and bounded by the ring under consideration — weight 
was taken and thickness determined in several series. When weights were to be 
taken, the shells were first heated in an electric oven to a temperature of 95° to 105° C. 
for an hour before the actual weighings were made. The readings were taken to the 
nearest 0.1 gram. In the case of weight determinations, only left valves were used 
for the sake of uniformity, as the two valves are not quite the same weight, owing 
to the differences in the teeth along the hinge margin. Thickness determinations 
were made by means of steel bow calipers, operated with a milled screw. As the 
females are readily distinguishable in the species of Lampsilis by the greater convexity 
