18 TERRESTRIAL MOLLUSCA INHABITING SOCIETY ISLANDS. 
are erroneous, and have, in consequence been a fruitful factor in the introduction of 
synonymous species. Several years after Cuming’s visit, the naturalists of the United 
States Exploring Expedition, commanded by Capt. Wilkes, collected a number of new 
species, all of small size, which were described by Dr. Gould in the “Proceedings of 
the Boston Society of Natural History,” and subsequently more elaborately described 
and figured in the official work, ‘* Mollusca and Shells.” In 1854, MM. Hombron and 
Jacquinot (Voy. Pol Sud), described two new species and added a synonym to 
Gould’s Helix Cressida, two to Pfeiffer’s Helix coarctata, and one to Gould’s Helix 
bursatella. In 1867, Johann Zelebor, one of the naturalists of the ‘ Novara” 
expedition round the world in 1857 to 1859, described Pupa hyalina (= Vertigo 
pediculus), Pupa Dunkeri (= Vertigo tantilla), and Hydrecena Scherzeri, all found on 
Tahiti; the last probably equals one of the extreme forms of the variable Omphalo- 
tropis scitula. 
During the years 1860 to 1863, I made a much more thorough exploration than 
any of my predecessors, and, by searching in nearly every valley in the group, dis- 
covered over 50 new species. Most of these were described by the late Mr. W. H. 
Pease in the “ Proceedings of the Zoological Society,” and in the “American Journal 
of Conchology.” The other species with his MS. names have been freely distributed, 
and the majority recorded in catalogues. -All of these are for the first time described 
in the following pages. Since my residence in the group, from 1870 up to the present 
time, I have continued my researches, and added 19 new species to the list, one of 
which, Partula acuticosta, Mousson, MS., is recorded in “* Museum Godeffroy Catal., v,” 
and one, Partw’a Mooreana, Hartman, is described in the “ Proceedings of the 
Academy of Natural Sciences of Philadelphia.” 
Genus MICROCYSTIS, Beck. 
I restrict this genus to a group of small Helices, which are characterized by their 
orbicular, more or less depressed form, rounded, angulate or subangulate periphery, 
and smooth, shining surface. The umbilicus, though usually closed, is occasionally 
minutely perforated. The peristome is straight and sharp, with remote margins. 
The columella is simple, or callous, and frequently armed with a nodule or slightly 
twisted plait. In color they vary from whitish corneous, through all the intermediate 
tints, to fulvous; rarely ornamented with bands and spots. One species only exhibits 
a sculptured surface. 
They are widely diffused throughout Polynesia, ranging from the lowlands near 
the seashore to several thousand feet above sea-level. A majority of the species are 
strictly terrestrial, and delight in moist stations, hiding beneath decaying leaves, under 
rotten wood and among loose stones. Others are entirely arboreal, on the foliage of 
shrubs and ferns. A number of the species are gregarious. 
