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 than 500 feet higher, with many others nearly as lofty in the vicinity.  Calippus has not apparently a central peak or any other features on the floor.

CASSINI.—­This remarkable ring-plain, about 36 miles in diameter, is very similar in character to Posidonius.  It has a very narrow wall, nowhere more than 4000 feet in height, and falling on the E. to 1500 feet.  Though a prominent and beautiful object under a low sun, its attenuated border and the tone of the floor, which scarcely differs from that of the surrounding surface, render it difficult to trace under a high angle of illumination, and perhaps accounts for the fact that it escaped the notice of Hevel and Riccioli; though it is certainly strange