In the eastern part of the _Mare Fecunditatis_ lies a pair of relatively small craters named Messier, which possess particular interest because it has been suspected, though not proved, that a change of form has occurred in one or other of the pair. Maedler, in the first half of the nineteenth century, represented the two craters as exactly alike in all respects. In 1855 Webb discovered that they are not alike in shape, and that the easternmost one is the larger, and every observer easily sees that Webb's description is correct. Messier is also remarkable for the light streak, often said to resemble a comet's tail, which extends from the larger crater eastward to the shore of the _Mare Fecunditatis_. Goclenius and Guttemberg, on the highland between the _Mare Fecunditatis_ and the _Mare Nectaris_, are intersected and surrounded by clefts, besides being remarkable for their broken and irregular though lofty walls. Guttemberg is forty-five miles and Goclenius twenty-eight miles in diameter. The short mountain range just east of Guttemberg, and bordering a part of the _Mare Nectaris_ on the west, is called the Pyrenees. The _Mare Nectaris_, though offering in its appearance no explanation of its toothsome name--perhaps it was regarded as the drinking cup of the Olympian gods--is one of the most singular of the dark lunar plains in its outlines. At the south it ends in a vast semicircular bay, sixty miles across, which is evidently a half-submerged mountain ring. But submerged by what? Not water, but perhaps a sea of lava which has now solidified and forms the floor of the _Mare Nectaris_. The name of this singular formation is Fracastorius. Elger has an interesting remark about it. "On the higher portion of the interior, near the center," he says, "is a curious object consisting apparently of four light spots, arranged in a square, with a craterlet in the middle, all of which undergo notable changes of aspect under different phases." Other writers also call attention to the fine markings, minute craterlets, and apparently changeable spots on the floor of Fracastorius.