1911.] NATURAL SCIENCES OF PHILADELPHIA. 131 



one hand, and dry conditions, with poor food .supply and generally 

 poor cover, on the other. Favorable conditions as to shade and even 

 as to the character of vegetation (as absence of palms, etc.) may yet 

 produce small forms where there is a deficiency of moisture. Such a 

 case is the sink hole back of the barbecues on the Somerset place, 

 where the forms were rather small because the rock was not honey- 

 combed with holes, and hence the cover was not very good. The 

 conditions of moisture were not the most favorable on account of the 

 overhanging walls, but still the sink was not especially dry. But 

 food was not plentiful and cover was rather lacking. Even the 

 P. a. goniasmos was small, as was the P. jamaicensis also; but the very 

 fact that this species was found at all, shows that the conditions were 

 rather favorable. Observation on a great many localities on this 

 estate shows that, as regards the distribution of the various sizes ©f 

 any one species, this is dependent on the character of their habitats, 

 which may be roughly classified as follows: 



A. Deep gullies and cavernous or honeycombed rocks, giving good 

 cover, abundant food and moisture, produce large well-fed forms. 

 This region is confined to the uncleared valleys and gullies and the 

 bases of the hills here, but may extend to hill-tops also, as at the small 

 hill noted on this estate and the higher hill at the Ridge near Lincoln. 



B. Slopes of hills rising from the more level ground, disturbed woods 

 and isolated hills wooded to the top not covered by the palmetto, yet 

 in virgin forest or bush where the conditions as to shade are favorable, 

 but the rock cover not so good as in A and the food supply not so 

 plentiful, produce the average medium-sized forms in great numbers. 



C. Hill-tops, especially those covered by the palmetto called " broom- 

 palm," where the rock cover is wanting and food is not plentiful and 

 where the conditions are dry except after rains, have few and small 

 forms. 



In the condition C there is generally another unfavorable feature 

 present, namely, the nests of the large black ant, which in such places 

 builds nests of 7 or 8 feet high and 3 feet in diameter on the ground 

 or in trees. In many cases these nests or ant cities are very old and 

 have exercised an influence for years. Near such ants' nests no shells 

 were seen dead or alive, although sometimes dead shells were encoun- 

 tered within 50 to 100 feet of such nests. The white termites (called 

 locally "duck-ants") also build large dome-shaped nests, but more 

 often on the slopes, and I did not see that they had much influence on 

 the distribution of the molluscs. 



