360 



GEOLOGICAL SURVEY OF THE TERRITORIES. 







United States, Pennsylvania, 

 Virginia, North Carolina. 















Estheria minuta (Albert!) var. brodieana 

























Hanover, Germany, England. 







Keuper f 





France, Germany, 



Bunter ) 





















Saxony. 







Tipper Carboniferous . . 



Leaia leidyi (Lea) var. ^pllliamsoniana Jones.. 

 Estheria tenella (Jordan) 



England. 



France, England, Germany. 













Lower Carboniferous . . 



Estheria striata ( Miinster) var. tateana 



England. 





Entheria striata var. binneyana 



En "-land. 







Pennsylvania. 

























It appears from the foregoing table tliat the oldest Phyllopod crusta- 

 cean is a genuine Estheria,* judging, of course, from the carapace valves 

 alone ; tlie more or less problematical form, Leaia, being carboniferous. 

 Thus Estheria dates from the Devonian. 



As to the ancestral forms of Phyllopods in general, they may have 

 been derived from forms like the Cladocera. Limnetis indicates in its 

 resemblance to Daphnia, that from this Branchiopod with its cladocerous 

 allies the Phyllopods may have sprung. Next below the Branchiopods 

 stand the Copepoda from which all the other Neocarida have sprung; 

 the Copepoda all originating from a nauplius ancestor. The Ostriicoda, 

 the lowest suborder of Branchiopoda, flourished in the Lower Silurian 

 seas, hence the Branchiopoda must have originated in the Laurentian 

 period and the Phylloi^od suborder at least as early as the Upper Silu- 

 rian period. 



The accompanying table may serve to give a rude idea of the rela- 

 tions of the principal groups of tbe Crustacea, and their appearance in 

 geological history, so far as the extremely scanty data we possess will 

 allow, while the diagram may also serve as a genealogisal tree, showing 

 the probable origin of the main divisions of the Crustacea. 



As is well known, the Trilobites are met with in comparative abund- 

 ance in the lowest fossiliferous beds of the Silurian period, and they are 

 the most ancient of Crustaceans, so far as their remains give evi- 

 dence. The genera Conocephalites, Dicellocephalus, Paradoxides, and 

 Agnostus, besides other forms, appear in the Potsdam sandstone or 

 equivalent i>riraordial rocks of this and other countries. The type dis- 

 appeared during the Carboniferous period, the genera Phillipsia (one 



*As there exists some doubts in my mind as to the Estherian nature of JS. palex 

 Clark, I have left this out of present consideration. 



