Suggestive in several ways for its resemblance to the carets is another detail not far distant from the Portus Sigaei, the twin-forked Sabaeus Sinus. Curiously enough, this feature of Mars, which has been well known and recognized ever since the eagle-eyed Dawes detected it more than forty years ago, proves to be a sort of connecting link between the main markings and the details of more modern detection. The twin-forked Sabaeus Sinus, as its name implies, is of the form of a double bay; was considered to be one in fact so long as the _maria_ were held to be seas. It straddles the point of land which, called the Fastigium Aryn, has been taken for the Greenwich of Martian longitudes. Each ‘bay’—not in truth a bay at all—indents the ochre in an acute triangle, from the tip of which many canals proceed like the rays of a fan from a holding hand. Both tips are darker than the main body of the dark _mare_ from which they proceed. They thus recall in general character the carets. They further reproduce specifically the Portus Sigaei, for they give origin to two doubles, the Gihon and the Hiddekel, in exactly the same manner that the two nicks of the Portus Sigaei do to the Phison and Euphrates. Nor are their tips much farther apart than those of the Portus, five degrees measuring the spread of the one and four degrees that of the other respectively; the reason for their earlier discovery lying in their greater size. They thus perform the same office as the Portus Sigaeus, are quite comparable to it in width, and differ in shape only as a larger and more acute triangle differs from a smaller and blunter one. Undreamt of by Dawes and unheeded since, they were the first hint to the world of the duality which forms so strangely pervasive a feature of the canal system of the planet.
Thus the carets stand connected with the canals quite as intimately as the oases but in a significantly different manner. For, in addition to their intermediary standing between the light regions and the dark, their relation to the doubles is peculiar. An instance is offered by the double Euphrates and another by the Ganges. The Euphrates, as we saw in Chapter XVIII, leaves the Portus Sigaei at the south, one line leaving each caret centrally, so that each caret is concerned only with its own line and has no connection with its fellow. At their northern ends both lines have similarly each its own Lucus Ismenius. The like seems to be true of the Ganges. Similarly the twin Titan, have each its own. Such twin duty in the matter of doubles seems to be the rule with the carets, even more so than with the oases; and this is probably from the fact that the coastline is of more limited extent than the interior.