CLINTON GROUP. 
63 
BRACHIOPODA OF THE CLINTON GROUP. 
The very considerable number of species of Brachiopoda which occur in this group, and 
which are not found elsewhere, make it desirable to study and illustrate them separately. The 
species which are known to ascend into the Niagara group are generally inconspicuous in 
this group, being few in number, while the typical forms are abundant. The most conspicuous 
species is the Pentamerus oblongus , and the most numerous the little Atrypa hemispherica , and 
almost equally so the A. congesta, all of which seem remarkably restricted in their vertical 
range. 
Regarding the limited thickness of the group, and the circumscribed area which is accessible 
along its line of outcrop, the Brachiopoda are really very numerous, showing in fact a group of 
species not to be mistaken or confounded with those of any other group. With the exception 
of an inconsiderable number, the species are strongly contrasted with those of the lower period; 
and this fact will become more apparent as we study them more carefully. On the other hand, 
we at once perceive the introduction of types which assimilate the species of this group with 
the succeeding ones, and which are eminently typical of the Middle Silurian period. 
We observe at this period a gr.eat diminution in the number of species of Orthis, while the 
individuals are likewise rare : the forms known are, with one exception, nearly equivalved, and 
we have few of those with coarge striae. 
In the leptaenoid type we find at this period, for the first time, the introduction of those 
forms with a crenulated hinge-line (the Genus Stropheodonta). The remarkable form 
Chonetes, is also first introduced at this period, and, with the preceding genus, continues 
throughout the system. 
The species of Atrypa are comparatively more numerous than Leptjena or Orthis, when 
compared with the preceding period. In this genus we perceive the introduction of the smooth, 
rounded or subcylindrical forms, which can scarcely be said to occur in the lower period of the 
system. We also recognize/in several species, characters which are observed in a single species 
in the lower period, and which proves the necessity of a separation among the forms now 
grouped under this genus, into several genera. 
Lastly we have -the introduction of the Genus Pentamerus, which is wholly unknown in the 
preceding period, and which eminently typifies the middle portion of the series in this country. 
Already we know about seven or eight species, all of them occurring between the Clinton group 
and the Onondaga limestone inclusive, while not a single one is yet known in any higher or 
lower position. 
