CLINTON GROUP. 



53 



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 »/3. 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 great 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 coarse striae. 



In the leptsenoid 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 Lept^na 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 

 pre"ceding 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. 



