45 
fauna. A study of the table reveals the gradual change from 
one fauna to the other. 
About nineteen species common to both the Black River 
and the Trenton are shown in these three sections. Two are 
rare in both formations; five are relatively abundant in both, 
though some are more frequent in one or in the other; five are 
common in the Black River and rare in the Trenton; and eight 
are rare in the Black River and abundant in the Trenton. Of 
the remaining species, forty-nine do not appear above the Black 
River beds, and sixty-seven are confined to the Trenton. Of 
the forty-nine Black River specimens seventeen are found 
abundantly. Eight of these may be regarded as typical horizon 
markers for this district. These index species of the Black 
River are Columnaria halli Nicholson, Stromatocerium rugosum 
Hall, Pianodema subaequata (Conrad), Rafinesquina minne- 
sotensis N. H. Winchell, Cyrtodonta canadensis Billings, Bucania 
halli Ulrich and Scofield, Actinoceras bigsbyi Bronn, Bumastus 
milleri Billings. A ninth species, Hormotoma gracilis (Hall), is 
common in these beds and does not appear in the lower 
Trenton section though it is found in higher Trenton beds at 
Hull and elsewhere, suggesting that it is a recurrent species. 
Of the sixty-seven species confined to the Trenton beds twenty 
are abundant, and of these twenty some of the most distinguish- 
ing horizon markers are Prasopora simulatrix var. orientalis 
Ulrich, Triplecia extans Hall, Plectorthis pectinella (Hall),* 
Parastrophia hemiplicata Hall, Sinuites cancellatus (Hall), 
Bucania punctifrons Emmons, Phragmolites compressus Conrad, 
Hormotoma trentonensis Ulrich and Scofield, Hormotoma belli- 
cincta (Hall), Encrinurus cybeleformis Raymond, Calymene 
senaria Conrad, and Ceraurus dentatus , Raymond and Barton. 
