ELISHA MITCHEI.L SCIENTIFIC SOCIETY. 73 



are to be taken account of, namely, (i) the age at which 

 reproduction commences, (2) the frequency with which 

 broods are produced, (3) the number contained in each 

 brood, and (4) the length of time during which the bring- 

 ing forth of broods continues. 



Accordingly, for the special case of Physa heterostropha 

 we have the followino- results: 



1. Age at which reproduction begins, 5 months. 



2. Frequency of broods, i in about 2yV days. 



3. Number in each brood, 30 average. 



4. Reproductive period, 4 months, March to July. 

 Some addition ought to be made to this actually observed 



period, inasmuch as the snails had certainly already entered 

 upon it at -the time of their capture, and, further, instead 

 of closing normally, it seems to have been violently inter- 

 rupted. Just how much the period of reproduction is to 

 be extended I have no means of determining, unless the 

 fact that the young snails of the first brood were observed 

 reproducing themselves in September warrants an exten- 

 sion of at least two months, making it six months instead 

 of four. "^ 



Assuming, then, that the reproductive season extends 

 from March to September, and assuming, further, some- 

 what arbitrarily, that the snail lives but two years, we have, 

 on the basis of facts above mentioned, the followino^ esti- 

 mate of the total number of the offspring of a single pair: 



At close of first season 1,900 



950 pairs at close of second season 1,805,000 



Original pair at close of second season 1,900 



Total number offspring in two 3'ears 1,808,800 



*Packard (Zooloo^-, p. 266) states that the "egg-s of P. heterostropha are laid in the 

 early spring, and three or four weeks later from fifty to sixty embryos with well-formed 

 shells may be found in the capsule." The apparent inference that only a single brood 

 is produced must of course be dismissed. 



W.AKE Forest College. X. C. 



