﻿98 PROCEEDINGS OF THE NATIONAL MUSEUM vol. loa 



I. 1 mile up La Sa Fua River. May 10, 1945. R. T. Abbott collector. 20. 



Flow sluggish, 4 feet wide, 2 inches deep ; shady. 

 J. y-i mile up Inarajan River. May 2, 1945. R. T. Abbott collector. 43. 

 K. Merizo River. 1945. D. Frey collector. 245. 

 L. Small stream near Ajino Beach. 1945. D. Frey collector. 8. 

 M. y-2 mile up Umatac River, May 10, 1945. R. T. Abbott collector. 36. 



Flow rapid. 30 feet wide, 8 inches deep ; no shade. 



The snails are most abundant in the shallow riffles where the flow 

 of water is fairly rapid and where the bottom consists of a pavement 

 of small stones. Occasionally the snails may be found congregated 

 on firm sandy bottom provided there is a healthy flow of water. Waters 

 of the stream that are exposed to direct sunlight during most of the 

 day appear to attract this species. The upper reaches of the streams 

 that have a very rapid flow of water are not favorable, although other 

 moUusks such as Neritin-a and Septaria may be abundantly represented. 

 The tiny flows of headwater tributaries, which are less than a foot 

 in width and 2 inches in depth, will not support T. granifera but are 

 favorable to T. scabra. The stream at Umatac, in the southwest end 

 of Guam, has the colony with the greatest number of largest individ- 

 uals. The maximum concentration of snails (about 10 per square 

 foot) is found in the fairly level section of the stream, which is about 

 30 feet wide and half a foot deep. This area is only a few hundred 

 feet from the high-tide mark of the ocean but is probably never diluted 

 with salt water except perhaps during typhoons, which may come from 

 the west. 



Conditions at Agana Spring, the most northerly located colony on 

 the island, are interesting in the dwarfing effect on the size of the 

 individuals. The spring is used as a source of water for nearby Agana 

 and at present has a pumping station located there. The spring 

 proper is about 30 feet deep, with a diameter of about 60 feet. The 

 upsurge of water is moderate, but the overflow is relatively fast, con- 

 sisting of a 100-foot sluiceway about 3 or 4 feet wide. There are 

 abundant algal growths in the spring. The shells and animals of this 

 colony are similar to those of other Guam populations except in the 

 reduction in size. No environmental cause for this could be found. 

 Even more pronounced dwarfing is evident in specimens that were 

 introduced to the large spring at Lithia, Fla. 



On Leyte Island, Philippine Islands, where numerous collecting sta- 

 tions were made, Thiara granifera was found in large rivers and small 

 streams. High in the central mountains, where large stream condi- 

 tions duplicate those on Guam, colonies of this snail are rather com- 

 mon. In the extensive Leyte Valley, at the northeast end of the island, 

 the larger rivers, with silty to sandy bottoms, meander; only rarely 

 is T. granifera found along the very edges of the rivers, where the flow 

 of water is moderately fast. However, smaller streams, which pass 

 down from the foothills bordering the valley, are excellent habitats. 



