J67. CONTRIBUTION TO ^ 



to be composed of gix small plates surrounding an elongate rectangular openings 

 the mouth perhaps. 



The specimen is laterally flattened as is usually the' ease with specimens of 

 this genus. 



The base has no scar for stem attachment and the specimen was probably 

 sessile. 



The plates are all a little convex especially noticeable at the center. On- 

 some plates there are nodes and warty prominences. Pores are scattered rather 

 thickly over the plates. In outline our specimen differs from all described 

 species, as well as in the number of plates, but here is a genus with no definite 

 number of plate rows nor plates in a row. Even rows can not sometimes be traced 

 and disorder is often occasioned by the intercalation of small plates between the 

 larger ones. The base has a stem or nothing but a fiat face for attachment^ 

 roots (S. A. Miller) or the base is rounded. 



In other words, some of the species are attached by stems, others by roots, 

 still others are sessile or free floating. 



Some species have scars for the attachment of arms? or spines, usually four 

 in number, while others have no indication of such appendages. 



It would be hard to state upon what characters this genus is founded and 

 what would be good specific characters. 



A slight change in form, as lengthening or shortening the longitudinal di- 

 ameter, increases or decreases the number of rows of plates and plates in a row, 

 and these things furnish the paleontologist an excuse for the creation of many 

 species. Of the thirty odd species of the genus, perhaps over half came from 

 practically the same locality, and it would be safe to say that eight or ten spe- 

 cies could be made to include the whole thirty-five or more. 



In a genus of such variation, should we not expect considerable variation 

 in specimens of the same species? 



Niagara group, Big Creek, near Dupont, Jeff'erson county, Ind. All of the 

 fossils figured on this plate and described in this paper are the property of Mr. 

 G. K. Greene. 



