nsisting of a depression partially enclosed by mountain arms.

SIRSALIS.--The more westerly of a conspicuous pair of ring-plains about
20 miles in diameter, in the disturbed mountain region some distance S.W.
of Grimaldi. It has lofty bright walls, rising to a great height above a
depressed floor, on which there is a prominent central mountain. The E.
border encroaches considerably on the somewhat larger companion, which
is, however, scarcely a third so deep. One of the longest clefts on the
visible surface runs immediately W. of this formation. Commencing at a
minute crater on the N. of it, it grazes the foot of the W. _glacis_;
then, passing a pair of small overlapping craters (resembling Sirsalis
and its companion in miniature), it runs through a very rugged country to
a ring-plain E. of De Vico (De Vico _a_), which it traverses, and, still
following a southerly course, extends towards Byrgius, in the
neighbourhood of which it is apparently lost at a ridge, though Schmidt
and Gaudibert have traced it still farther in the same direction. It is
at least 300 miles in length, and varies much in width and character,
consisting in places of distinct crater-rows.

CRUGER.--A regular ring-plain E. of Fontana, 30 miles in diameter, with a
dark floor, without detail, and comparatively low bright walls. There is
a smaller but very conspicuous ring-plain (Cruger _a_) on the W. of it,
to which runs a branch of the great Sirsalis cleft.

EICHSTADT.--A ring-plain, 32 miles in diameter, near the E. limb, S. of
Rocca. It is the largest and most southerly of three nearly circular
enclosures, without central mountains or any other details of interest.
On the W. lies a great walled-plain with a very irregular border,
containing several ring-plains and craters, and a crater-rill. Schmidt
has named this formation DARWIN.

BYRGIUS.--A very irregular enclosure, about 40 miles in diameter, between
Cavendish and the E. limb, with a lofty and discontinuous border, rising
at one point on the E. to a height of 7000 feet above the floor. There
are wide openings both in the N. and S. wall, and some ridges within. The
border is broken on the E. by a crater, and on the W. by the well-known
crater Byrgius A, from which a number of bright streaks radiate, mostly
towards the E. One on the W. extends to Cavendish, and another to
Mersenius, traversing the ring-plain Cavendish C. North-east of Byrgius
there is a mountain arm which includes a peak 13,000