26 THE NATTrLUS. 



No method has been discovered for determiniug the age of 

 young or the period of incubation. I am inclined to believe 

 that 3'oung in Calyculina are carried for one year or more. In 

 a pond wliich was under observation for a period of over a year, 

 adults were found in April to contain fully formed young. This 

 l)ond had been dry from July of the year preceding until it 

 became filled by melting snow in March. It had no inlet or 

 outlet and received no overflow floods from any nearby ponds 

 or streams. The number of young produced is jirobal)ly ten to 

 twenty' in Calyculina and two to four in Sphaerium. These 

 figures are based on the fact that these species during earlj' 

 spring contain about the above numljcrs of young, which when 

 removed from the mother are able to take care of themselves. 



Age in Sphaerium. The distinctness of growth-areas or rings 

 in Sphaerium led to the belief that age might be determined 

 by correlating size and number of rings. The entire surface of 

 the shell (PI. VI, Fig. 10) is thrown into narrow parallel ridges 

 which represent growth-lines. At the beginning of a season 

 these lines are widely separated; at the end they are crowded 

 very closely together. One of these areas constitute a growth 

 ring. In many specimens the several rings are further marked 

 by slight differences of color due to deposits on the shell. The 

 rings are usually very distinct though there are many forms in 

 which they are very faintly differentiated. 



The following table includes results obtained Ity mensuring 

 Sphaerinw simile. 



Total rings 



1 

 2 

 3 

 4 

 5 

 6 

 7 

 8 



