Sir Roderick Murchison divided his great Silurian system primarily into two parts, Upper and Lower. This method of classification is generally followed even at the present day, although it is widely recognized that the most decided break in the entire Palaeozoic group is the one between these divisions. In 1879 Professor Lap worth proposed to give due emphasis to this distinction by erecting the Lower Silurian into a separate system, the Ordovician. The name is taken from the Ordovici, an ancient British tribe which dwelt in Wales during Roman times. Lap-worth's example is now largely followed in England and the United States, but on the continent of Europe the name Silurian is still retained for both systems.

The classification and subdivision of the American Ordovician were first worked out in the State of New York, and consequently the New York scale serves as a standard of reference for the rest of the continent.

In the preceding table the classification lately issued by the New York Survey is given in comparison with that of the Welsh Ordovician. It is not to be supposed, however, that the subdivisions in the two continents are exactly equivalent, but merely that they correspond to one another in a general way.