1871.] DAEKNESS AjSTD HICKS ST. DAVId's PKOMONTOKY. 395 



With reference to the distribution in time of the earlier genera of 

 Trilobites, it would appear that the genus Ohniis is represented in Bri- 

 tain and Europe by twenty species^ Tvhich are confined to the horizon 

 of the Lingula-flags, but not occurring so low as the Meneyian 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. undulostnatus, 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 com- 

 mon to Britain and Texas, C. depressa, Salter ; and this, in "Wales, 

 makes its appearance in the Tremadoc slates ; and in America an- 

 other form, G. ZenJcerl, 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 Mlcrodiscus one form is mentioned by Dr. Dawson 

 as occurring in the primordial rocks of New Bruns'wick, 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 BiTceloceplialus 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 po- 

 sition in America than in Britain. 



