1911.] natural sciences of philadelphia. 159 



Variation in the Two Groups. 



At Somerset, in passing from the base of the hills or from the valleys 

 to the hill-tops, the form is observed to change, and the rise is quite 

 noticeable as the tops of the hills are reached. From Somerset to 

 the Ridge near Lincoln the rise in the spire is still continued, and this 

 latter locality is a hill-top. At Garrett's woods and at Bloomfield 

 the spires are higher than at any of the Somerset stations and as high 

 or higher than at the Ridge near Lincoln. In the other series, from 

 Kendal Road to the woods at King Edward Hotel, there is a similar 

 rise in the spire. This change in the form cannot be correlated with 

 change of altitude measured from sea level, but is directly connected 

 with a diminution in size ; as the size of the shell diminishes the shape 

 changes, and the ratio of height to width increases, or the spire rises. 

 Examined locally, as at Somerset, this change may be seen in passing 

 from the base to the top of a hill, so that it may be stated that valley 

 forms and those living at the base of the hill are lower in the spire 

 than those living at the hill-top. The only place where valley and 

 gully forms were seen in a natural, undisturbed condition was at 

 Somerset. In passing from the gullies and shaded valleys, with their 

 wealth of rank tropical vegetation and the ground covered with a 

 thick moist layer of dead leaves, to the stony hill-top, with its hardwood 

 trees and broom-palm, the number of specimens encountered became 

 greatly reduced. On some hills at Somerset, scarcely a ground- 

 living shell, such as Pleurodonte, was seen at the top of the hill, yet 

 the shade of the woods was often very dense. But an examination 

 of the ground showed that the surface coating of leaves was dry, it 

 was mostly the fallen leaves of the broom-palm, which decay very 

 slowly and do not form a good material for the growth of the fungus 

 upon which the Pleurodontes appear to feed. The palm leaves form 

 a sort of thatch on the ground and shed the water on the hill-top, 

 hence the leaves are dry ; and the loose stone on most of these hill-tops 

 makes a porous layer, through which what water penetrates the palm 

 thatch soon drains away. Comparing these Somerset hills with those 

 examined elsewhere in the more cultivated regions, the comparatively 

 small number of individual specimens encountered upon the hill-tops 

 was very noticeable. But the molluscan population throughout all 

 of this undisturbed region on the Somerset plantation was much less 

 dense than in the majority of places that were examined in the more 

 disturbed regions, and probably represents the original condition in 

 these more cultivated parts. In such places as the localities in the 

 cleared and cultivated region near Mandeville, as Garrett's woods. 



