TRENTON CONGLOMERATE OF RYSEDORPH HILL 97 



Trenton in Minnesota. Isochilina armata Walcott, var. 

 pygmaea is a variety of a species which has been found by its 

 discoverer only in the Lowville and Black river limestone of Rus- 

 sia, Herkimer co., while Primitia mundula and A p a r - 

 chites minutissimus are later forms with prenuneial 

 varieties in the Trenton. The evidence afforded by these fossils 

 is obviously strongly indicative of lower Trenton age of the com- 

 pact black limestone; for it must be considered that the great 

 majority of the forms begin in the Lowville and Black river lime- 

 stone and rise into the Trenton, while few begin in the Trenton 

 and have their principal development in younger beds; some 

 forms, as Streptelasma corniculum, Callopora 

 multitabulata, Zitteloceras hallianum, and 

 Isochilina armata var. p y g m a e a , can be regarded as 

 restricted to the lower Trenton, or having their principal develop- 

 ment there^ The general appearance of the fauna of the compact 

 black limestone pebbles, is hence, such as points more to a close 

 relation with faunas older than the Trenton, than with younger 

 faunas, and is, in a general sense, indicative of lower Trenton 



age. 



Keddish gray and gray pebbles 



A comparison of the faunas of the compact reddish gray 

 (group 6) and of the gray crystalline limestone (group 7) pebbles 

 ehow^s that the two differ only in their ostracode element; the 

 reddish gray limestone contains only a few other fossils, all of 

 which, with the exception of Ampyx hastatus, Gera- 

 saphee ulrichana, Pterygometopus calliceph- 

 a 1 u s , occur also in the gray limestone. The ostracodes peculiar 

 to the reddish gray limestone are either new species or new 

 varieties, with the exception of Eurychilina reticulata. 

 As, further, lithologic transitions from one to the other occur in 

 the same pebble, it is apparent that both are derived from adjoin- 

 ing or alternating beds, and that it will serve our purpose to 

 treat both faunas together. 



Of the large number of species identified in these pebbles, 

 the new species and Rafinesquina alter nata, on 



