THE NAUTILUS. 27 



While the above results are not conclusive they are certainly 

 not altogether negative. The following facts seem significant. 



1. No individuals attain a length of over 19 mm. 



2. No individuals show more than 8 rings. 



3. The averages of each class show an increase in length of 

 about 2 mm. per ring. 



4. The range of size included within each class is from 5 to 

 7 mm. This seems to indicate either that growth is very irreg- 

 ular in different individuals or that the rings do not indicate 

 seasons. The fact that many specimens have very indistinct 

 lines may account for some of these discrepancies. If one ring 

 represents a year, the average age of a large specimen would be 

 four or five years. 



Regarding growth in the Unionidae Isely (1913) draws the 

 following conclusions. 



1. Rate of growth is exceedingly variable. 



2. The summer months are the growth months. 



3. Lines of arrested growth may be called rest rings, the con- 

 spicuous ones being usually winter rest rings. Occasionally 

 the rest rings may be two or more years apart ; more often, 

 however, several equally prominent rings may be formed in one 

 year. Prominent rest rings are generally due to double pris- 

 matic and epidermal layers. 



Notes on Ecology. So far as observed the food consists mainly 

 of diatoms. Many forms occur in ponds which become dry 

 during the summer, remaining in that condition until the follow- 

 ing spring. In aquaria I have observed that Cal3^culina will 

 burrow down to the water level. Isely reports forms of Union- 

 idae which were turned up by a plow in perfect condition. 



The above observations emphasise the fact that very little is 

 known about the habits of the forms considered. 



BIBLIOGRAPHY. 



1828 — Jacobsen, Cycladens anatomiske. 



Undersolgelse Dansk. Selsk. Naturvid. Afhandl, Vol. 



Ill, translated by T. Prime. 



Bulletin of the Museum of Comparative Zoology, Vol. 5. 



