176 



Contributions to Palceontology. 



rently begin their existence somewhere in this central epoch, 

 but their precise relations to the other beds have not been 

 determined. In the higher beds of the formation, and 

 clearly separated from the great central mass, we have the 

 Genera Dikelocephalus, Triakthella and Aglaspis, 

 together with Lingula, Serpulites and Euomphalus. We 

 observe, therefore, that the earliest trilobites are referable 

 to the Genus Conocephalites ; and the Genus Dikeloce- 

 phalus does not appear in the first stages of the formation, 

 nor below the beds which I have referred to the second or 

 middle stage of the period. There this genus appears in 

 three species, smaller and less conspicuous than those in 

 the higher beds. It is only in the later stages of the sandstone, 

 that the typical species of this genus of Dr. Owen appear; 

 and those from the lower beds, thus referred by him, belong 

 apparently to other genera. 



There is much yet to be done in the middle and lower 

 beds of the formation, both in the way of determining the 

 range of species and genera, and in the study of new or 

 imperfectly known species. I believe, however, that the 

 specific and generic relations here indicated may serve to 

 guide investigation ; and as the localities are often isolated, 

 the position in the series may be determined by a little 

 attention to these remarks, and a comparison of the cha- 

 racters of species and genera. 



Comparatively few species have been determined from 

 authentic Potsdam sandstone in its more eastern localities, 

 although numerous forms have been described from calca- 

 reous and shaly beds, regarded by some geologists as equiva- 

 lent, by others as newer, and by still others as older than 

 the Potsdam sandstone. I have made no comparisons with 

 these, since they appear to me all entirely distinct from 

 those which I have enumerated and described from the 

 sandstone of the Mississippi valley, and approached in 

 character only by the later forms of that period. 

 In this connexion, moreover, our attention is directed to 



