PORTAGE GROUP 
231 
These forms are too well known to require description, but they often assume some fan¬ 
tastic shape which causes them to be mistaken for an organic body. This arises from the 
seams on the surface, which are fancied to resemble the lines of suture in the shells of the 
tortoise or turtle, and by this name they are frequently known in the neighborhood where 
they occur plentifully. From the great number of these in some parts of this group, they 
frequently join each other, and assume many of the singular forms which we meet with in 
“ clay-balls,” or concretions in ordinary clay, which, however, rarely attain more than a few 
inches in diameter. 
The following form is so unique that it seems worthy of illustration, as showing the imita¬ 
tive power of concretionary force. 
99. 
Concretion or Septaria, one third the natural size, 
