CONTKIBTJTIONS TO PALjEONTOLOGT. 183 



CONCLUSION. 



Although I have not been able to recognize the successive 

 Trilobite beds of the Sandstone as indicated by Dr. Owen, I can 

 nevertheless refer the species here described to three different 

 epochs in the Potsdam period ; and I am not prepared at the pre- 

 sent time to suggest any farther subdivision. In the lower beds of 

 the formation I have found Conocephalites proper, together with 

 LiNGULA, LiNGULEPis, Obolella? and Theca. In the middle stage, 

 neither the limits of the beds, nor the range of species or genera, 

 have been so well determined ; but grouping together all that I 

 have found between the well-defined upper beds and the lower 

 fossiliferous beds known, we have Conocephalites, Dikelocepha- 

 Lus, Arionellus, Ptychaspis, Chariocephalus, Ill^nurus and 

 Agnostus, in the trilobitic fauna, together with Orthis and Pla- 



TYCERAS. 



The Graptolitid^ apparently 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, Tri- 

 arthrella and Aglaspis, together with Lingula, Serpulites and 

 Euomphalus. 



We observe, therefore, that the earliest trilobites are referable 

 to the Genus Conocephalites ; and the Genus Dikelocephalus 

 does not apper 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 

 characters of species and genera. 



