330 
PACIFIC SCIENCE, Vol. IX, July, 1955 
occur also in northern Bougainville or Buka. 
Of the nine non-endemic forms, one, Opeas 
gracile , is a cosmopolitan tropical form which 
has probably attained its present wide distribu- 
tion through the assistance of man. Five spe- 
cies, Omphalotropis nebulosa , Leptopoma vitreum, 
Pseudocyclotus levis , Eustomopsis eustoma , and 
Pupina keraudreni , occur both in the Bismarck 
Archipelago and throughout the Solomons. 
Partula carteriensis is known elsewhere only 
from New Ireland. Syncera nitida guamensis 
occurs in the Carolines and Sturanya modesta 
has a fairly wide distribution throughout the 
Solomons and New Hebrides and has been 
recorded from Samoa. The two best repre- 
sented elements in the fauna therefore are 
(a) the four endemic forms, and (b) the five 
species that range from the Bismarck Archi- 
pelago to the southern Solomons. The latter 
group must consist of species that either have 
been in the area for a long period of time or 
have efficient methods of dispersal. Since such 
geographical variation as has been observed 
in members of the group in this area is very 
slight or incipient, it seems most unlikely 
that these forms have been long established 
on Nissan or the other islands of the Solo- 
mons. We have, then, the situation that the 
two groups of species best represented on 
Nissan consist of one that has been present 
for a sufficiently long period to acquire strong 
differentiating characters and another that has 
become widely spread in the Solomons and 
colonized Nissan en route. That this move- 
ment has probably been mainly from the 
northwest to the southeast is shown by the 
wide distribution of such forms as Leptopoma , 
Pseudocyclotus and chloritid mollusks in the 
Austro-Malayan region. 
The waifs and strays of the Nissan fauna 
are such as one would expect in an oceanic 
island, and three of these, Paludinella , Syncera 
and Opeas , are forms that appear to be con- 
fined to coastal areas. 
Nissan is an elevated and probably tilted 
coral atoll, some 15 miles in diameter. Most 
of the surface of the island is covered by 
dense forest apart from two quite considerable 
patches of coconut plantations. Situated be- 
tween Buka in the northern Solomons and 
New Ireland it is about 30 miles from Buka 
and 70 miles from New Ireland. However, it 
is more closely connected with New Ireland 
by the small island group of Feni, which is 
about halfway between. Ocean depths be- 
tween Nissan and New Ireland are at least 
over 2,000 fathoms, and it is most unlikely 
that the two areas have been joined in the 
recent past by a fall in sea level. The land 
Mollusca of Nissan (apart from the endemic 
species which may be older in origin) must, 
therefore, have been derived by transoceanic 
migration. Very little is known of the methods 
used for such transoceanic dispersal by land 
snails although there has been a great deal 
of conjecture, especially in the literature of 
the latter part of the nineteenth century. What 
does seem very apparent is that some land 
snails do cross ocean barriers. At least it is 
as logical to assume that they do, as to pos- 
tulate a land bridge in the face of strong 
biological and geological evidence to the 
contrary. In an attempt to derive some ele- 
ments of the Solomon Island land snail fauna 
from the Bismarck Archipelago, the strait be- 
tween New Ireland and Nissan presents the 
first and probably the widest oceanic barrier. 
The land snails present on Nissan show that 
some species have crossed this gap compar- 
atively recently and that presumably they 
could also spread further south. 
These remarks would apply to the Eusto- 
mopsis , Omphalotropis , Leptopoma , Pseudocyclotus 
and Pupina. Partula carteriensis has apparently 
crossed the strait but has not, so far as is 
known, extended its range further south. 
Three of these forms, Leptopoma , Pseudocy- 
clotus and Partula are arboreal in habit while 
the other three are terrestrial, so there is ap- 
parently no advantage in motility as regards 
type of habitat. 
