63 



The above belong to the dominant family of the trilobitic 

 fauna of the Potsdam. 



Agnostus 3 " 



Amphion 5 " 



Ampyx 1 " 



Asaphus 4 " 



Cheirurus 8 " 



Endymionia 1 " 



Holometopus 1 " 



Harpes 1 " 



Harpides 1 " 



Illamus 5 " 



Nileus 1 " 



Lichas 1 " 



Remoplearides 1 " 



Shumardia 1 " 



73 



In this list I have placed all the species in the genera to which they 

 were originally referred, except the species of Loganellus, which was 

 described as an Olenus. The two species of Arionellus may possibly 

 belong to Prof. Hall's newly proposed genus Ptychaspis. In the genus 

 Bathyurus there are two species, (B. conicus and B. Cordai^) which evi- 

 dently have the characters of Owen's genus Crepicephalus. In Dikelo- 

 cephalus Sesostris, we have also a form which I consider to be perfectly 

 congeneric with D. Miniscaensis , one of the types of Ptychaspis. Meno- 

 cephalus is not recognized by Hall and Shumard among the fossils of the 

 Potsdam sandstone of Wisconsin, where the specimen on which Owen 

 proposed the genus was procured. Crepicephalus is also placed by them 

 in Conocephalites, where, indeed, it is perfectly admissible, provided the 

 great extension given to this latter genus be sustained. I think, however, 

 the number of species is becoming so great that, sooner or later, Conoce- 

 phalites will be broken up into a number of genera. Be this as it may, we 

 have, in the Lie* vis formation, the leading generic types of the dominant 

 family of the Potsdam trilobites, although (in accordance with the views 

 of different authors) not all under the same names. All the species (39 

 in number) that I have referred to Arionellus, Bathyurus, Bathyurellus^ 

 Conocephalites, Dikelocephalus, Menocephalm and Loganellus, belong to 

 that family. Comparing the trilobitic fauna of the Potsdam with that of 

 the Levis formation, we thus find that in the former, thirteen out of seven- 

 teen genera, and fifty-three out of sixty species ; and in the latter, seven 

 out of twenty-one genera with thirty-nine out of seventy-three species 

 belong to the same zoological group. This shows that, while this peculiar 

 type of trilobites strongly connects the two faunae, during the period of 

 the Le>is formation it was on the decline. 



