﻿100 PROCEEDINGS GF THE NATIONAL MUSEUM 



Table 3. — Data pertaining to collections from Lithia Spring, Fla. 



1 Insufficient numbers of specimens to ascertain maturity point or significant mean of shell length. 



Table 3 shows the results obtained from 11 sample collections taken 

 from Lithia Spring, Hillsborough County, Fla., in February 1947. 

 The stations are marked on the map of the spring (fig. 41) . Stations 

 6, 8, 9, and 11 had insufficient numbers of specimens to ascertain a 

 reliable point at which maturity is reached, or a significant mean of 

 shell length. 



The map has been marked with short arrows, which indicate the 

 main flow of the current of water coming from the spring and with 

 long, broken arrows, which indicate the flow of river water that 

 breaks across the low land to the north of the spring during flood 

 periods. The river water, which is 18° C. in contrast to the 25° C. 

 of the spring water, is fatal to these snails and undoubtedly accounts 

 for their absence or low numbers at stations 8, 9, and 11. These 

 last-mentioned stations are well stocked with local Florida snails 

 (pleurocerids, amnicolids, and planorbids). 



The mean shell-length data given in table 3 is of little value in 

 making a comparison of two populations, and, in fact, can be very 

 misleading. A look at the population-growth curves of shell length 

 (fig. 42) will reveal the fallacy of the mean shell length. Most of the 

 populations give a bimodal curve, as accentuated in population 4. The 

 first peak is simply an aggregation of immature specimens, a factor 

 that will vary according to the reproductive cycle of that population 

 and that will change in value as the yoimg grow to an adult size. In 

 population 4, maturity is reached at 6.5 mm. and extends up to 12.5 

 mm. The mean for these adults, which are continually gi'owing, is 

 about 7.5 mm., yet if the entire population, both immature and adult 

 specimens, is averaged, the mean must be recorded as 5.5 mm. This 

 difficulty may be avoided in great measure by comparing only adults. 



