18 
THE MEDITERRANEAN NATURALIST 
which is constantly illuminated. He carried our 
a continuous series of observations on the spots 
of Mercury by means of the new large refractor 
which has recently been installed at Milan. With 
regard to the rotation of the planet, he finds that 
the motion of Mercury round the .sun is similar to 
that of our moon round the earth, and that it 
always presents .the same hemisphere to the sun. 
It, however, possesses a greater flibration of lon- 
gitude.” Hence three-eights of the planet’s sur- 
face is continually in the blaze of the sun, and an 
observer in this region sees the sun oscillate in 
the sky over an arc of 47°, the double oscillation 
taking a period of 88 earth-days. Another three- 
eighths of the planets surface is turned away 
from the sun, and is consequently in continual 
darkness, being only illuminated by refracted rays 
and twilights. In the intervening tract of one 
quarter of the surface there is a single alteration 
day and night during, each interval of 88 days, the 
length of the day and of the night varying at each 
place according to its position, but constant for 
the same place. 
The possibility of the existence of organic life : 
will depend upon the existence of an atmosphere 
capable of distributing the sun’s warmth. Schia- 
parelli thinks he has discovered indications of an 
atmosphere in white clouds appearing as bright ! 
spots, and rendering the image of the spots indis- 
tinct. 
He supposes the dark spots to be tracts of water, 
and these he finds are not aggregated into large 
tracts forming oceans, but appear to branch and 
ramify through the land as canals. Such an alter- 
nation of land and water he thinks would cause a 
more complete equilibrium of temperature.. The 
peculiarity in the rotation of Mercury is an excep- 
tion among the planets, but common among the 
satellites of the planets. Mercury has no satellite, 
so that it presents a remarkable divergence from 
the prevalent condition among the planets. 
Natural Science in Tunis. 
The scientific explorations of Tunis, since it 
came under the dominion of France in 1881, says 
the Contemporary Review , is apparently making 
good progress. Two volumes, dealing with some 
the results already obtained, have recently 
reached us, both cf which are deserving of rh e 
attention of geologists. In one of them we have 
descriptions of a series of fossil mollusca, obtained 
from some of the cretaceous formations of the 
country, and in the other a set of plates illustra- 
ting a portion of the formations met with. 
The specimens dealt with were collected in 1885 
and 1886 in the region which lies to the south of 
the elevated plateau of Tunis, and from the stand- 
point of the paleontologist, are of considerable 
value and importance. In an introduction which 
precedes the technical descriptions, M. Peron 
briefly discusses the chief features of the creta- 
ceous fauna of Tunis, lays down the principles 
which have guided himself and his collaborateur 
in the making and determining of species, and 
corellates his conclusions with those obtained 
from a study of the cretaceous rocks of other 
countries. 
As might have been expected from the conti- 
nuity and similarity of geological structure which 
they exhibit, Tunis and Algeria have a similar 
fossil fauna, and hence the work done on the 
paleontology of the latter country by M. Coquand 
has materially assisted the others in the task they 
have undertaken. 
But while admitting to the full the value of the 
assistance rendered them by M. Coquand, both by 
his writings, and in other ways, we think they are 
justified in claiming the credit of having advance 
considerably beyond the position he attained, and 
of having introduced something like order into a 
subject which was somewhat chaotic. Here as in 
many other cases, species and genera have been 
founded on insufficient material; individual varia- 
tions have not been allowed for, and the determi- 
nations of earlier investigators have been ignored. 
Hence the difficulties met with by the others in 
the determination of the species and the interpre- 
tation of their relations to previously described 
forms were both real and considerable. These 
difficulties were especially felt in dealing with the 
Gasteropoda , most of which had to be reclassified, 
even the generic character in some instances hav- 
ing been misconceived or confounded. 
Perhaps the greatest service they have rendered 
to paleontology, however is the great reduction 
they have been able to effect in the number of 
species of Ammonites., Plicatula and Ostrea, a 
