e wall is terraced within, and has a crater just below
its crest on the W., which, when the opposite border is on the morning
terminator, is seen as a distinct notch. Autolycus is the centre of a
minor ray-system.

ARISTILLUS.--A larger and much more elaborate ring-plain, 34 miles in
diameter, N. of Autolycus. Its complex wall, with its terraces within,
and its buttresses, radiating spurs, and gullies without, forms a grand
telescopic object under a low sun on a good night. It rises on the east
11,000 feet above the Mare, and is about 2000 feet lower on the W., while
the interior is depressed some 3000 feet. Its massive central mountain,
surmounted by many peaks, occupies a considerable area on the floor, and
exhibits a digitated outline at the base. On the S. and W. a number of
deep valleys radiate from the foot of the border, some of them extending
nearly as far as Autolycus. Shallower but more numerous and regular
features of the same class radiate towards the N.E. from the foot of the
opposite wall. On the N.W. are several curved ridges, all trending
towards Theaetetus. On the S.E. the surface is trenched by a number of
crossed gullies, well seen when the E. wall is on the morning terminator.
Just beyond the N. _glacis_ is a large irregular dusky enclosure with a
central mound, and another smaller low ring adjoining it on the S.E. The
visibility of these objects is very ephemeral, as they disappear soon
after sunrise. Aristillus is also the centre of a bright ray system.

THEAETETUS.--A conspicuous ring-plain, about 16 miles in diameter, in the
Palus Nebularum, N.W. of Aristillus. It is remarkable for its great
depth, the floor sinking nearly 5000 feet below the surface. Its walls,
7000 feet high on the W., are devoid of detail. The _glacis_ on the S.W.
has a gentle slope, and extends for a great distance before it runs down
to the level of the plain. Not far from the foot of the wall on the N. is
a row of seven or eight bright little hills, near the eastern side of
which originates a distinct cleft that crosses the Palus in a N.W.
direction, and terminates among mountains between Cassini and Calippus. I
have seen this object easily with a 4 inch achromatic.

CALIPPUS.--A bright ring-plain 17 miles in diameter, situated in the
midst of the intricate Caucasus Mountain range. On the E. is a brilliant
peak rising more than 13,000 feet above the Palus Nebularum, and nearer
the border, on the N.E., is a second, more tha