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Dawes was a superb observer, for sure. The original drawings for 1864-65 that were used to prepare the engravings for that RAS publication are held in the Mars Section archives. The engravings are faithful reproductions. If you look at the bottom right drawing on the plate there do appear to be four forks, but the one nearest the limb is actually Margaritifer Sinus. In high resolution maps (such as Ebisawa’s, uploaded to the Section website) there is the usual partly resolved broad fork, with lighter shading between, and then a third ‘prong’ on the following or W. side, the latter (which Ebisawa called Brangaena) very thin leading to a very small dark spot at the north end. This can be seen in good current images of the region and I have seen it visually this year. It is of course harder to discover something than for a later observer to confirm it! In 1941 the Pic du Midi observers resolved the westernmost prong of the Meridiani Sinus into two closely spaced prongs, also shown on the Ebisawa map, though it is not a constant feature. So that would make four prongs. Historically there were always large differences in the drawings of this region, with some observers resolving two well-defined forks and others seeing the Sinus Sabaeus-Meridiani region like a sort of meat cleaver.
Although Dawes saw the planet briefly at the next opposition, these views of 1864-65 were his last ones. His excellent series from 1852 was reproduced in a small engraving by Proctor, and used in his well-known map, but they have never been reproduced from the originals.
It is odd that Dawes himself never tried to chart the planet.