236 The AmericoM Naturalist. [April, 
berry, and a locality three miles west from Mason City, on Willow 
Creek. 
This serial, judging from its lithological character and order 
of sequence, appears to be the equivalent of the " Genesee Shales " 
of the New York section, and to which division we would here 
refer it. 
As we have before intimated, the base of the Hamilton, represent- 
ed by the blue Shales at Independence, carries a rich Fauna, and 
evidence, also, of the former existence of a rich, and perhaps varied, 
flora, which was restricted to this zone. 
Of the fossil species occurring in this serial, the following may be 
Strophodonta arcuata Strophodonta variabilis 
Strophodonta calvini Strophodonta canace 
Strophodonta reversa Orthis infera 
Atrypa reticularis Atrypa hystrix 
Spirifera subumbonata Rhynchonella ambigua 
Gypidula munda Productus dissimilis 
Lepidodendron and Sigillaria 
Also several other undetermined species of Brachiopoda, and 
corals, and one or two species of crinoids. 
Of the above list of species, only two, Atrypa reiiailaris, and A. hys- 
trix, are known to occur in the Corniferous limestones below, while 
only three or four forms are at present known to extend upward into 
the middle Hamilton, (the shales, limestone, etc., lying above the 
blue shale and below the blue clay). 
The two species Atrypa reticularis and A. hystrix, as they occur in 
the overlying rocks, assume a form so altered as to be as readily dis- 
tinguished as if they were distinct species. The number of blue 
shale species which occur in the shales at Rockford, is greater than 
those of all the other divisions combined. A peculiar feature of 
this blue Shale Fauna, is the depauperation of most of its spe- 
As to the flora of this division, it is, as we have before stated, in- 
digenous to it ; none of the other serials containing any evidence of 
the former existence of either terrestial or marine plant life. 
The rocks of the middle Hamilton carry a rich and varied fauna, 
more particularly in its lower and central portions. 
