_Grimaldi._ An immense walled plain, extending over 148 miles from N. to S. and about 130 miles from E. to W. Its interior is very dark. _Clavius_ is another grand example of this class of object, and is rather larger than Grimaldi, but unfavourably placed near the S. pole. _Schickard_ may also be mentioned as a large formation of similar type, and situated near the S.E. limb of the Moon. [Illustration: Fig. 27. Birt, Birt A, and the Straight Wall. 1883, Feb. 15, 6^h to 8^h 40^m. (T. Gwyn Elger.)] _Rill or Cleft of Hyginus._ A conspicuous example of the lunar rills, and one which yields to very moderate instruments. Neison notes that it is readily visible in a 2-inch telescope; while Webb remarks that a power of only 40, in a good instrument, is enough to show it under any illumination. The rill is about 150 miles long. It cuts through a number of crater-pits, and Mädler found so many widenings in it that it appeared like a confluent train of craters. The rill traverses the large crater-pit Hyginus, which is 3-3/4 miles in diameter and moderately deep. Other fine examples of rill-systems will be found between Rheita and Metius and near Triesnecker and Ramsden. _Straight Wall._ A singular structure on the E. side of the ring-plain Thebit. It is a ridge or wall, which looks regular enough for a work of art, according to Webb. Its average height is 450 feet (Schröter), 1004 feet (Mädler), or 880 feet (Schmidt). These several determinations are given to show the discordances sometimes found in the measures of good observers. This object is about 60 miles long; at one extremity lies a small crater, at the other there is a branching mountain nearly 2000 feet high. Elger has drawn this object, under both a rising and a setting sun, in the Liverpool Astronomical Society’s ‘Journal,’ vol. v. p. 156, and remarks that it may be well observed at from 20 to 30 hours after the Moon’s first quarter. _Valley near Rheita._ South of the ring-plain Rheita, on the S.W. limb, there is an enormous valley, which extends in its entire length over 187 miles, with a width ranging from 10 to 25 miles. There are several fine valleys in this particular region. _Leibnitz Mountains._ These are really situated on the further hemisphere of the Moon, but libration brings them into view, and they are sometimes grandly seen in profile on the S. margin. Four of the peaks ascend to elevations of 26,000 or 27,000 feet, and one mass, towering far above the others, is fully 30,000 feet in height, and is unquestionably the most lofty mountain on the Moon.