The first round coinage

The round coins were cast for the first time at the end of the Zhou . The round coins were much more handy, than the earlier types of coins, and replaced the heavy and clumsy knife and spade coins. Since they have been excavated together with knife coins, we know that they coexisted for a while. They were small and had a hole, so they could be carried in a string. Some of the types had a round hole, some had a square hole. Many numismatists believed earlier, that the square-holed coins was of post-Qin origin, but we know now that there were coins with round holes in the Warring States period.
The round coins were used in both the spade- and knife areas. The dating of the first round coins are still very uncertain, but Wang gives us several different views and reasons, that it should be around 250 B.C. (p. 202). The round coins of Zhou were abolished by the Qin ruler in 221 and replaced by the Ban Liang. The round coins were thus cast over a relatively short period of time, and this must be the explanation why there have been excavated so relatively few of them. We have a few common types today, but for most of the mints there remains only a few specimens.
Since the round coins replaced the knives, some of them were given the same name as the knives of their area, and from this we know where these coins originated. 

The round coins of the Knife area. Hua round coins have been unearthed together with Ming knives in Shandong, and was thus from the Qi state (Peng Xinwei p. 80 and Wang p. 189-190). All had square holes and the monetary unit hua .

1. The Yi Hua round coins came from the city Yi of the Qi state. They had square hole, plain reverses and the same denominations as the knives: one, two, four and six Hua. Some of the Yi Hua had rims, some not.

2. The Ming Hua round coins had square holes and plain reverse, but no rim. The character ming is the same as on the Ming knives. Nearly all catalogues list the Ming coins as Ming Dao , but Wang states that the character which looks like dao was in fact an abbreviation of Hua , namely the left part of the character in ancient script. This abbreviation was already used on the small knives and the Yi Hua round coins. It has never been misread on the knife coins and Wang wonders why it has been misread on the Ming coins (p. 190-191). The denominations of the Ming Hua were one and four Hua, possibly also 2.

3. The Yi Hua had square holes, rims and plain reverse. The legends consisted of the numeral one and the character hua. It is uncertain if there were other denominations than one. 

The round coins of the spade area. This group can be divided in 5 types:

1. Round hole with no rim.
2. Round hole with rim.
3. Square hole with no rim.
4. Square hole with rim.
These four types appeared in this sequence. 

5. The monetary unit of these coins were the liang , which was also the unit on the late spades of the Qin state. The appearance of this unit on these coins implies that they developed from the Qin spades. Among the denominations of this type, there where the Zhong Shi Er Zhu . One liang were 24 Zhu, 12 zhu was a liang (see weights). This coin must have been the forerunner for the Ban Liang of the Qin empire, where coins were standardized throughout the whole country (p. 195-196).

Further reading on early round coins:
Coole, Arthur Braddan: Earliest round coins of China. 1981.
This book has very many types.

Peng Xinwei pages 37-40. (Peng calls round-hole coins ring-coins).