When silver money was struck, it was natural that the barter-unit which came nearest in value to the silver didrachm would be equated to it, and the piece of silver would accordingly be termed _Shield_ or _Tortoise_, just as the silver equivalent for the old copper rod was called the Obol, and in due course the corresponding device would be impressed on the silver coinage. The same explanation may probably be applied in other cases, such as that of the boar on the coins of Lycia. On the coins of the Gaulish tribe Sequani who made the best bacon and hams which came into the Roman market, the swine is found[408]. Doubtless this animal was their chief source of wealth, and formed a unit of barter, but we have not space for any more examples. It is worth noting that it is quite possible that the men who issued the earliest coins of Boeotia and Aegina were influenced in the shape they gave these coins by the actual objects which they were replacing. The coins of Aegina with their high round upper side and flat under side suggest the general outline of a tortoise. As the people of Olbia, like the Chinese, Burmese and Ceylonese, had to make coins in the shape of a fish, so the Aeginetans acting under a like instinct may have wished to give a conventional representation of the tortoise. The earliest coins have the incuse on the reverse divided into _eight_ triangular compartments. Are these the _eight_ plates which form invariably the _plastron_ or under surface of all the tortoise family? Later on the Aeginetan incuse is always in five compartments, but in the two well-known triangular depressions we perhaps find an echo of the tortoise-_plastron_[409]. The earliest coins seem to represent a sea-tortoise, for the feet are real _flippers_ quite distinct in shape from the legs shown on the later coins. As the plates of the _carapace_ (upper surface) are not fully represented in the archaic coins, this omission may not be merely due to rudeness of work, but rather because in the case of the sea-tortoise the _thirteen_ plates of the _carapace_ are not so prominent as in the land-tortoise. On the later coins where the feet are those of the land-tortoise the coins accurately represent the _thirteen_ plates.