s of
a once complete N. wall.

BILLY.--A ring-plain, 31 miles in diameter, S.E. of Letronne, with a very
dark floor, depressed about 1000 feet below the grey surface on the W.,
and a regular border, rising more than 3000 feet above it. There is a
narrow gap on the S., and indications of a crater on the N.W. rim. Two
small craters stand on the S. half of the interior. The formation is
flanked on the S.W. by highlands.

HANSTEEN.--A somewhat larger ring-plain, with a lower and more
irregular rampart, rising on the W. to nearly 3000 feet above the
floor, which is depressed to about the same extent as that of Billy.
Both the inner and outer slopes are terraced on the E., where the
_glacis_ is traversed by a short, delicate, rill-like valley.
There are some bright curved ridges on the floor. On the W. of Billy
and Hansteen is a wide inlet of the Oceanus Procellarum, bounded by
the Letronne region on the W., and on the S. by lofty highlands. On the
surface, not far from the S.W. border of Hansteen, is a curious
triangular-shaped mountain mass, with a digitated outline on the S., and
including a small bright crater on its area. Between this and the ring-
plain is a large but somewhat obscure depression, N. of which lies a
rill-like object extending from the N. point of the triangular mountain
to the W. wall. At the bottom of a gently sloping valley between Billy
and Hansteen is a delicate marking, which seems to represent a cleft
connecting the two formations.

ZUPUS.--A formation about 12 miles in diameter with a dark floor,
situated in the hilly region N.E. of Mersenius.

FONTANA.--A noteworthy ring-plain, about 20 miles in diameter, E.N.E. of
Zupus, with a bright border, exhibiting a narrow gap on the S. and two
large contiguous craters on the N.W. The faint central mountain stands on
a dusky interior. On the N. is a large peculiar depressed plain with a
gently sloping wall, within which are three short rill-like valleys and a
crater.

MERSENIUS.--With its extensive rill-system and interesting surroundings,
one of the most notable ring-plains in the third quadrant. It is 41 miles
in diameter, and is encircled by a fine rampart, which on the side
fronting the Mare Humorum rises 7000 feet above the floor, which is
distinctly convex, and is depressed 3000 feet below the region on the E.,
though it stands considerably above the level of the Mare. The
prominently terraced border is tolerably regular on the N.W., but on the