With reference to the distribution in time of the earlier genera of Trilobites, it would appear that the genus Olenus is represented in Britain and Europe by twenty species,, which are confined to the horizon of the Lingula-flags, but not occurring so low as the Menevian group, and by two species which, in Wales, are found in the higher horizon of the Tremadoc slates. In America there are four forms of this genus, and these are found in positions higher than any of the Old- World species ; and the American forms are distinct from those of Britain and Sweden. Three of these species occur in the Quebec series ; and one 0. undulostriatus, Hall, has been obtained from the Hudson-river group.
Of the genus Conocoryphe (Conocephalites) there appear to have been found in the Potsdam sandstone of America twenty-six species ; and if to them we add the fourteen forms obtained by Dr. Dawson from New Brunswick, forty species of this genus seem to occur in the New World, all being low down in the primordial rocks. There is one form not included in this number, which appears to be common to Britain and Texas, C. depressa, Salter ; and this, in Wales, makes its appearance in the Tremadoc slates ; and in America another form, C. Zenkeri, Bill., is found in the Quebec group.
In the Old World eighteen species of the genus Conocoryphe occur among the representatives of the Lingula-flags and the underlying deposits, and four forms make their appearance in the Tremadoc slates.
In America this genus does not range so high as the genus Olenus ; and in the same country it has been met with in a lower horizon than Olenus.
Of the genus Microdiscus one form is mentioned by Dr. Dawson as occurring in the primordial rocks of New Brunswick, and one form appears also in the Quebec group.
Wales affords two species of this genus, one of which is found in the Menevian series, and the other in the purple rocks of St. David's. This genus also ranges higher in America than in the Old World.
As regards Arionellus, four species have been recognized in the Potsdam sandstone of America, and two have been obtained from the Quebec group. In Britain, and on the continent of Europe, three forms occur ; and these are found low down among the earlier rocks.
This genus also appears in a higher horizon in America than in the eastern hemisphere.
Of the genus Dikelocephalus there are in the Potsdam sandstone of America twelve forms ; and in the Quebec group thirteen species occur. In Britain, which seems to be the only country where this genus occurs in the Old World, there are four forms ; and these have a limited range, three being confined to the Upper Lingula-flags, and one to the Tremadoc slates.
Here, again, we have a genus which is represented in a higher position in America than in Britain.
