August 6, 1920] 



SCIENCE 



135 



movement a small knob, K, is screwed to each 

 of tlie movable sticks. A slider, S is made 

 like a small T-square. It crosses the sticks 

 at a right angle and can be moved freely 

 along them. The two guides are slightly- 

 thicker than the meter sticks so that the 

 movement of the slider does not change the 

 position of the movable meter sticks. M is 

 used in determining the age of the male and 

 F that of the female. For the sake of sim- 

 plicity the millimeters are not shown in the 

 figiu-e. 



The method of using this device is best 

 illustrated by an example. Suppose the male 

 of a pair was bom on April 10, 1919, and 

 the female on February 19, 1919. The first 

 date is the 100th day of the year, and the 

 second date is the 50th day. A calendar 

 having all the months of the year on one page 

 and also having each day in the year num- 

 bered consecutively from both the beginning 

 and the end of the year is used for deter- 

 mining what day in the year a certain date 

 represents. Stick M is moved until its right 

 end is even with the 100-millimeter mark on 

 A and F is similarly moved to the 50-milli- 

 meter mark. If a weighing were made on 

 June 9, 1919, which is the 160th day of the 

 year, the slider is moved to the 160-millimeter 

 mark on A. The age in days of each animal 

 is now indicated on their respective sticks by 

 the number of millimeters to the right of the 

 slider. That is, the male is 60 days old and 

 the female 110. If the age is to be computed 

 on November 27, 1919, the 331st day of the 

 year, the slider is moved to 331 on A and the 

 age of each at once read off, which is 231 and 

 281 days respectively. The ages at any date 

 in 1919 can thus be computed without moving 

 anything but the slider. 



If a date occurs in the succeeding year, 

 1920, then the sticks would require resetting. 

 This is done in the following manner. The 



slider is moved to the 365 mark on A, which 

 represents the last day of 1919, and the read- 

 ings taken on M and F. These are 265 and 

 315 respectively, that is, the ages on Decem- 

 ber 31. These two nmnbers may also be 

 found on the calendar since they are the days 

 in the year when numbered consecutively 

 from December 31 to January 1 correspond- 

 ing to the two dates of birth. M is now 

 moved to the right until its 265 millimeter 

 mark is even with the end of A and F is 

 moved in a similar manner until its 315 mark 

 is even. This arranges the instrtiment for 

 any date in 1920. If the ages are desired on 

 March 28, 1920, the 87th day of the year, the 

 slider is moved to 87 on J. and the ages of 

 the two animals are at once indicated as 

 352 and 402 days respectively. In this man- 

 ner the ages may be rapidly determined for 

 any date desired. It is obvious also that 

 the device can be arranged to give the ages 

 when the two animals are born in different 

 years. 



The limit of capacity of this device is 1,000 

 days. But in dealing with animals with a 

 longer span of life each millimeter can repre- 

 sent a week, a month, or a year and the ages 

 computed in these periods of time. 



The excuse for this article is the hope that 

 it may help some one who is confronted with 

 a series of tedious computations similar to 

 the ones with which I have had to contend. 



J. ROLLIN •SlONAKEK 



Physiology Depaetment, 

 Stanford TjNrvERSiTY 



THE CLASSIFICATION OF THE OPALINID.ffl 



I HAVE completed a study of one hundred 

 and thirty-four species and twenty subspecies 

 which according to the prevalent usage would 

 be included in the genus Opalina. Twenty- 

 four species were known before. My material, 

 obtained mostly from the United States Na- 



