﻿108 PROCEEDmCS OF THE NATIONAL MUSEUM vol. 102 



This division into two groups has put all the few perfect specimens 

 in group A and has left us "adult" and all eroded specimens (except 1) 

 in group B for comparative studies. 



Group A Oroup B 



Total number 114(100%) 142(100%) 



Not eroded.... 31(27.1%) 1(0.7%) 



Eroded 83 (72.9%) 141 (99.3%) 



The means, their standard deviation, and the standard error of the 

 means were calculated for the following: 



Total Shell Length 



Mean 5 i« N 



Group A 6.59 1.42 0.13 114 



Group B 12.46 1.47 .12 142 



Length of Last Whorl 



Group A 4.40 0.29 0.02 114 



Group B 9.03 1.04 .09 142 



The correction factor (length/width of perfect specimens) of 1.51 

 was then applied to the measurements of the last whorls in each 

 group to compare theoretical total shell length and actual total shell 

 length. 



Reduction 

 Total Last whorl Last whorl X caused bp 



length length correction erosion 



A 6.59 4.40 6.64 0.03 



B... 12.46 9.03 13.63 1.17 



It will be noted that the reduction caused by erosion in group A is 

 extremely small. This is due to the youth of the group, which has 

 been exposed to erosion for a relatively short time. In fact, 27.1 per- 

 cent of these specimens were without a trace of spire erosion. In 

 group B, representing adults that have been exposed to acid waters 

 for considerable time, the reduction has been 1.17 mm., or 11.6 percent. 



PARASITOLOGY 



Trematode Parasites. — This species of fresh-water snail is of par- 

 ticular interest to parasitologists because of its ability to serve as an 

 intermediate host of numerous trematodes, three of which have been 

 known, in their adult stages, to parasitize man. Other genera and 

 species of the snail family Thiaridae also have this intermediate host 

 ability, and their presence in the Orient is responsible for large endemic 

 areas of the human lung fluke {Paragoninius westerTnani) . 



In the course of dissecting specimens from many localities, it was 

 noted that some colonies in the Philippine Islands were infected as 

 high as 20 percent with unidentified heterophyidlike cercariae. Dis- 

 section of specimens from the introduced colonies at Lithia Spring, 

 Fla., failed to show any trematode infection. 



