PTEROPODA. 
193 
In 1873 Hall and Whitfield described Hyolithes gibbosus (probably a variety 
of H. primordialis), from the Potsdam sandstone. 
In 1876 the writer designated and illustrated Hyolithes principalis, from the 
Upper Helderberg group, H. aclis and H. striatus, from the Hamilton group. 
In 1877 Dr. T. T. Barrett described H. centemialis, from the Lower 
Helderberg group. 
Two other species, H. triliratus and II. singulus are added to the list in the 
present volume. 
The American species of the genus Hyolithes, according to their respective 
authors, are distributed as follows: 
Hyolithes (Theca) aculeatus, Hall 
H. aclis, 
H. triliratus, . 
H. singulus, 
H. striatus, 
H. ligea, 
H. principalis, . 
H. centennialis, 
H. parviusculus, 
H. Americanus, . 
(= triangularis, 
H. communis, 
H. Emmonsi, . 
H. excellens, 
H. gibbosus, . 
H. gregarius, 
H. impar, 
H. primordialis, 
H. princeps, 
U 
u 
<< 
u 
a 
u 
Barrett . 
Hall . 
Billings . 
Hall) 
Billings . 
Ford 
Billings 
H. and W. . 
Meek and Hayden, 
Ford 
Hall . 
Billings 
Lower Carboniferous. 
1 
> Hamilton group. 
^ Upper Helderberg group. 
Lower Helderberg group. 
Hudson Biver group. 
> Potsdam sandstone. 
y 
In this enumeration of eighteen species some allowance may, probably, be 
necessary for repetition, owing to the studies having been made from incom¬ 
plete or insufficient material, and from the want of a comparison of collections 
from the several localities. As these are now recorded, we have nine species 
from the primordial zone, one species from the second horizon, one from the 
third; six species occur in the Upper Helderberg and Hamilton groups which, 
from the occurrence of many identical species of fossils, are regarded as 
25 
