58 Journal of the Mitchell Society [September 



ment was then applied to species in which growth rings were not 

 evident and the age was determined by measuring the length. It 

 seems possible that this calculation proved satisfactory for C. picta, 

 since Agassiz held this species under observation for ten years. By 

 this method he determined that egg-laying begins about the eleventh 

 year in C. picta, or when the turtle has reached a length of 80 mm. 

 On the basis suggested by Agassiz, a specimen of C. cinerea, 130 mm. 

 in length, would be considerably over twentj'^-five years of age. The 

 great majority, which begin laying when the carapace length is 150 

 mm., would be very old. In fact, Agassiz cites one case of C. hellii 

 as being "ver^^ old" with a carapace length of 155 mm. 



Through the kindness of Dr. A. S. Pearse, we have been able to 

 secure some data as to the growth in C cinerea. For the past few 

 years turtles have been caught, measured, tagged, and then returned 

 to Lake Mendota. Two such turtles which were caught and tagged 

 August 6, 1917, were again taken on September 12, 1919. Two others 

 that had been tagged September 24, 1919, were taken again; one on 

 May 8, 1920, and the other on June 6, 1920. The data concerning 

 these four turtles is given in the accompanying table: 



Date tagged. Bate retaken. Interim. Size a Size b Increase. 



Aug. 6,1917 Sept. 12, 1919. 2,1, 6. 130 mm. 138 mm. 8 mm 



Aug. 6, 1917 Sept. 12, 1919. 2,1, 6. 130 mm. 137 mm, 7 mm 



Sept. 24,1919 May 8,1920. 0,8,14. 91mm. 92 mm. 1mm 



Sept. 24,1919 June 6,1920. 0,9,12. 66 mm. 70 mm. 4 mm 



While these data are insufficient, it has been used as a basis and it 

 has been calculated that a turtle measuring 130 mm. in length would 

 be from eleven to fifteen years of age. If this be true, even approxi- 

 mately, then there is a difference in the growth rate of C. picta and 

 C. cinerea. 



The relation of the length of the turtle to the beginning of egg- 

 laying, based upon the data of 1919 is shown in the accompanying 

 table : 



Length. 115 130 135 138 140 145 150 187 



nun. mm. mm. mm. mm. mm. mm. mm. 



Total No. of specimens 4 4 4 5 6 8 4* 1* 



No. with small eggs present 4 2 .. .. 1 



Large eggs present but 



would not lay in 1920 4 .. .. 2 1 



No. that would lay in 1920 1 1 2 



No. having eggs in oviduct 3 1 4 3 



No. taken on nest 1 •• •• 1 ^ 



* Specimens of this size with no t-pRS found in oviduct. 



