316 A. E. Verrill— The Bermuda Islands. 728 
shores or around the borders of brackish swamps, belonging to 
Alexia; Pedipes, 2 sp., fig.66; Plecotrema,; Melampus, 5 sp.; Trunca- 
tella, 3 sp.; Onchidinm. These last were probably introduced by 
natural means. Of the truly terrestrial species there are 4 slugs and 
23 snails. Among the snails there is a single endemic genus (Peci- 
lozonites*), with three living and four fossil species, of which three 
—Thy- 
sanophora hypolepta ; x10. Figure 69.—Helicina convexa; x2. 67, 69, by 
A. H. V.; 68, by Pilsbry. 
are extinct. (See Part IV.) Two other species of snails are also 
supposed to be endemic, viz., Thysanophora hypolepta Pilsb. (fig. 
68), and Helicina convexa Ptr. (fig. 69), but these are closely allied 
to West Indian forms. The large slug ( Veronicella Schivelye 
Pilsb., fig. 84), known only from Bermuda, in its habits and local- 
ized distribution appears like an introduced species, but if so its 
origin is still unknown. 
The following native species of West Indian origin are supposed 
to have been introduced independently of human agency : 
Thysanophora vortex (Pfr.); Greater Antilles; Bahamas; South- 
ern Florida. Figs. 70, a, 6. 
* This genus is the most interesting one. Its largest species (P. Nelsoni) is 
extinct, but it occurs abundantly in the older cave-conglomerates and eolian 
limestones, sometimes in strata exposed only at low tide, thus showing that it 
lived on the islands before their partial submergence, and indicating the com- 
paratively great antiquity of the genus. Its nearest allies are now found in the 
eastern United States. The three living species are P. Bermudensis ; P. Reinia- 
nus ; P. circumfirmatus (fig. 67a, 67b). A variety of the first is abundant as a 
fossil in the later and softer limestones, often retaining very distinct bands of 
brown color. The fossil variety (zonata V., nov.) is rather larger with a thicker 
and firmer shell, larger umbilicus, and thicker callus than the living form. Both 
varieties vary considerably in height of spire, size of umbilicus, and color. For 
two series of comparative figures, see Part IV, Geology. 
