130 PROCEEDINGS OF THE ACADEMY OF [Feb., 



that migration of snails from one to the other would be possible during 

 wet periods. That such migration frequently takes place does not 

 seem very likely, but the possibility of it makes the entire tract prac- 

 tically one colony. Even on the same hill it is easy to find evidence 

 that much migration of the forms is not common, but that it occurs 

 in some cases there is good evidence. 



Collections were made in the gullies, sink holes and dissected caves 

 that represent the virgin conditions in the low ground; also in the 

 pastures where many forms live, and in the wooded hills, which are 

 everywhere covered by the original forest. More than 20 such hills 

 were explored with thin intervening valleys, an area of several hundred 

 acres. The lower ground and many of the hill slopes are underlaid 

 by the purer limestone, many of the cliffs on the hillsides consist 

 of it, as do the sides of the dissected caves and the sinks (cockpits), 

 and some of the tops of the lower hills. The higher hills are capped 

 by the marly limestone, which covers them with loose blocks of the 

 weathered porous stone, for the rocks are rarely in place here unless 

 the cliffs extend up to the hill-top, which they sometimes do. As a 

 rule, the hills are somewhat flat-topped. On such hill-tops the rocky 

 surface is largely covered by a vigorous growth of the tall, slender 

 palmetto, known as the " broom-palm," above which the hardwood 

 trees of many kinds extend their dense shade. While well-shaded, 

 these hill-tops are, nevertheless, dry on account of the porous nature 

 of the limestone and the deep coating of leaves of the palmetto, which 

 latter deca}^s very slowly and is very porous without forming much 

 humus. 



In contrast to this dry condition is the very damp character of the 

 sink holes and deeper gullies in the limestone which are so shaded 

 that even the tree ferns nourish there all the year round. Some of 

 the smaller hills, notably one to the west of the road near the 

 southern edge of the estate are composed of the pure limestone 

 which becomes honeycombed by the action of the weather, and 

 is so liable to develop small sinks and cockpits. In this little 

 hill just mentioned the vertical circular holes abound, many up to 

 10 feet in diameter. It was in one of these little cockpits that the 

 best specimens of the large extinct form of P. acuta goniasmos were 

 procured. Specimens as large as these were not found in the living 

 state, but many from the gullies and lower ground approximated to 

 their dimensions and shape. From a study of the distribution of this 

 species and observations on other species, it is evident that the size 

 is largely a question of moisture, good cover and abundant food, on the 



