32 The Molluscan Family Planorbidae 



[2] Helisovna trivolvis fallax (Haldeman) 



October 2, 1931, fifteen specimens of tliis race, collectecl near Cambridge, Massa- 

 chusetts, were recei\'ed from Dr. Harold A. Rehder, then in Harvard University (now 

 x\ssistant Curator of Mollusca, United States National Museum). Egg capsules were 

 observed February 6. 1932, one measuring 4.5 by 3.5 mm. and containing six eggs, 

 and one 5.6 by 7 mm. with thirteen eggs. Tlie eggs of both capsules passed through 

 their development and were hatched by February 12. On February 28 another capsule 

 was laid measuring 5.2 by 4 mm., containing ten eggs, but was eaten by Protozoa after 

 three days of development. March 4 a capsule was laid measuring 5 by 6 mm. with 

 twelve eggs, and on Maich 10 another capsule was laid, 3.5 by 5.5 mm., containing 

 eleven eggs. On March 13 a capsule containing nine eggs was observed. On March 19, 

 twenty-two young and five adult snails were counted in the aquarium, the young 

 ranging in size from 1.8 mm. to 3 mm. in diameter. 



An examination on May 22 revealed seventeen young snails living. All of the 

 adult snails were dead. The li\ing young snails measured as follows: 



2 specimens 2.5 mm. in diameter with 2 whoils. 



3 specimens 2.5 mm. in diameter with 2^/4 whorls. 

 1 specimen 2.8 mm. in diameter with 2% whorls. 



3 specimens 3 mm. in diameter with 2V2 whorls. 

 1 specimen 3.2 mm. in diameter with 2i/{> whorls. 



4 specimens 3.5 mm. in diameter with 3 whorls. 



1 specimen 3.8 mm. in diameter with 3 whorls. 



2 specimens 4 mm. in diameter with 3 whorls. 



On June 26, 1932, the seventeen young fallnx were still alive. On August 15 there 

 were fourteen living young snails which measured as follows: 

 1 specimen 2 mm. in diameter with 2 whorls. 

 1 specimen 3 mm. in diameter with 2 whorls. 



1 specimen 3.5 mm. in diameter with 2i/-> whorls. 



2 specimens 4 mm. in diameter with 2^2 whorls. 



5 specimens 4.5 nun. in diameter with 3 whorls. 

 2 specimens 5 mm. in diameter with 3 whorls. 



2 specimens 5 mm. in diameter with 3M whorls. 

 On October 23, 1932, only four young jaUax remained living: 



1 specimen 3.5 mm. in diameter with 2^/^ whorls. 



1 specimen 4.5 mm. in diameter with 3 whorls. 



1 specimen 5 mm. in diameter with 3 whorls. 



1 specimen 5 mm. in diameter with 3^4 whorls. 

 On March 1, 1933, these four snails were still living and measured as follows: 



1 specimen 4 mm. in diameter with 2^2 whorls. 



1 specimen 5 mm. in diameter with 3 whorls. 



1 specimen 5.2 mm. in diameter with SMi whorls. 



1 specimen 5.5 mm. in diameter with 3^/4 whorls. 

 On May 1, 1933, only three living snails remained: 



1 specimen 4.9 nun. in diameter with 3 whorls. 



1 specimen 5.5 mm. in diameter with 31A whorls. 



1 specimen 5.6 mm. in diameter with Sy^ whorls. 

 On June 25 one young snail remained which measured 6 mm. in diameter and 

 had three and one-half whorls. 



This small subspecies of HeUsoma trivolvis lived in the aquarium 495 days or 

 about seventeen months. It grew from a size of 1.5 mm. in diameter when hatched to 

 6 mm. when it died, and increased from one and one-fourth to three and one-half 

 whorls. Adult fallax measures 15 mm. in diameter and has five full whorls. The last 

 remaining young was, therefore, somewhat more than one-third grown. Full maturity 

 would i^robably have been attained the third year after hatching. 



Conclusions of this sort, in which length of life span is estimated from growth 

 rates in aquaria, are only tentative and are possibly subject to considerable error. 

 Very early experiments on snail development tended to show that the size of reared 

 snails is influenced by the volume of the water and the container in which they grew. 

 Conclusive evidence of rate of growth in natvu-e could be determined by measuring 

 representative samples, from normal habitats, taken periodically as practiced by Van 

 Cleave (1932) and his students in the interpretation of life history data. 



