VARIATION IN THE POSITION OF THE 
ADDUCTOR MUSCLES OF ANA- 
DONTA GRANDIS SAY. 
ELLIOT R. DOWNING. 
TunoucH the kindness of Mr. Frank C. Baker, curator of 
the Chicago Academy of Science, I have had the privilege of 
examining and measuring a number of exceptionally perfect 
shells of Amadonta grandis Say. These shells were taken 
in August, 1897, from “ South Pond,” Lincoln Park, Chicago, 
where they were closely associated as a colony. As the pond 
was thoroughly cleaned four years previously, the age of the 
oldest shells is definitely known. Not only do the annual 
limits of growth show, but internally the muscle scars and the 
paths of migration are very distinct (see figures). 
During part of the year shell growth occurs rapidly and 
muscle migration is also rapid ; again, growth nearly ceases, 
and the narrow dark band is deposited. During this slow 
growth the muscle is about stationary, and then the very 
distinct scar is formed. 
The questions which I wished to settle were : 
. Does the muscle in its migration move at a rate propor- 
ose to shell growth, so that the muscle | retains a fixed 
relation to the shell’s proportions? 
2. If not, how does it vary? 
I chose for this study the ratio, 
distance from the umbo to the muscle 
distance from the umbo to the shell margin 



In all the shells the separation of the anterior adductor from 
the anterior retractor is marked by a very distinct ridge, making 
a fine line which lies in the path of migration. The distance 
measured along this line on the curve of the shell, from the umbo 
395 
