These are some additional photos by Rolf Nijsse, kindly transmitted by Herman Cremers, taken during a recent trip to Puerto Rico, Luquillo Experimental Forest. They show Polydontes acutangula (Burrow, 1815), which is very similar to the species shown here but may be distinguished by its dark rim of the foot. N.B.: A recent preliminary report on the terrestrial snails of the island by Robinson & Field mentions this species as Parthena acutangula.
The classification of this group of snails (currently in Pleurodontidae) is to a large extent based on anatomical differences (see Wurtz, 1955).
The Luquillo Experimental Forest is a field station for ecological work, and snails have been the subject already for decades. A review may be found here.
Wurtz, C.B. (1955). The American Camaenidae (Mollusca: Pulmonata). – Proceedings of the Natural Sciences of Philadelphia 107: 99-143. [http://www.jstor.org/stable/4064483]