"We may therefore with some confidence assume that we are at liberty to fill up the gaps in the strata of Greek monetary history which lie between Homer and the beginning of coined money on the analogy of the corresponding strata in other regions. This assumption, resting on a broad basis of induction and confirmed by a good deal of evidence special to Greece and Italy, will be found to explain the origin, not only of weight standards in those countries, but of the types on the oldest coins, such as the cow's head of Samos, the tunny fish of Olbia and Cyzicus, the axe of Tenedos, the tortoise of Aegina, the shield of Boeotia, and the silphium of Cyrene" (pp. 49, 50).
Professor Ridgeway's view is that while mythological and religious subjects do occur on Greek coins, it can be shown that certain coins, even in historical times, were regarded as the representations of the objects of barter of more primitive times.
The tunny fish continually passes in vast shoals through the sea of Marmora from the Black Sea to the Mediterranean. A representation of this fish appears invariably on the electrum coins of Cyzicus. "We know that the articles which form the staple commodities of a community in the age of barter virtually form its money. In a city like Cyzicus, whose citizens depended for their wealth on their fisheries and trade, rather than on flocks and herds and agriculture, the tunny fish singly or in certain defined numbers, as by the score or hundred and the like, would naturally form a chief monetary unit, just as the stock-fish (dried cod) were employed in mediæval Iceland. Are we not then justified in considering the tunny fish, which forms the invariable adjunct of the coins of Cyzicus, as an indication that these coins superseded a primitive system in which the tunny formed a monetary unit, just as the kettle and pot countermarks on the coins of Crete point back to the days when real kettles formed the chief medium of exchange?
"But far stronger evidence is at hand to show that the tunny fish was used as a monetary unit in some parts of Hellas. The city of Olbia, which lay on the north shore of the Black Sea, was a Milesian colony, and was the chief Greek emporium in this region. There are bronze coins of this city made in the shape of fishes and inscribed +THU+, which has been identified as the abbreviation of +thunnos+, _tunny_. When we recall the Chinese bronze cowries, the Burmese silver shells, the silver fish-hooks of the Indian Ocean, etc., we are constrained to believe that in those coins of Olbia, shaped like a fish, we have a distinct proof of the influence on the Greek mind of the same principle which has impelled other peoples to imitate in metal the older object of barter which a metal currency is replacing. The inhabitants of Olbia were largely intermixed with the surrounding barbarians, and may therefore have felt some difficulty in replacing their barter unit by a round piece of metal bearing merely the imprint of a fish, while the pure-blooded Greek of Cyzicus had no hesitation in mentally bridging the gulf between a real fish and a piece of metal merely stamped with a fish, and did not require the intermediate step of first shaping his metal unit into the form of a tunny.