BR ACHIOPODA. 463 



Monomerella, all of which are confined to the Lower and 

 Upper Silurian rocks. 



The genera of this family are especially characteristic of 

 parts of the Upper Silurian, and especially of the so-called 

 " Guelph Formation " of North America. The differences 

 between them are founded mainly upon internal characters, 

 and as specimens usually occur in the state of casts of the 

 interior, their study is attended with unusual difficulties. 



