PLA T E XX. 



PAGE. 



Fi<r. i. Xystk acanthus mirabilis, St. J. and W" 458 



1 a. Side view, natural size; 1 b, view from front, showing disposition of tubercles along 



the anterior margin, reduced to one-fonrth natural size ; 1 c, portion of posterior 

 face, reduced to one-fourth natural size ; 1 d, transverse section in upper part of 

 the spine, partially restored ; 1 e, similar section near base, showing the walls of 

 the pulp-cavity distorted by pressure; If— I, various views of tubercles, enlarged; 

 1 /, tubercle from one of the middle rows near top of spiue ; 1 g, from the third 

 " row near top ; 1 h, large tubercle from one of the anterior rows in middle of the 

 spine; 1 i, large transverse tubercle near base; Ik, I, small tubercles of middle 

 rows near base. Coal No. 4 or 5, Fulton county, Illinois. 

 Pig. 2. Xystr acanthus ACiKACiFOitHis, St. J. and "W 459 



2 a. Side view of spine, natural size, with enlargements of tubercles ; 2 b, view of poste- 



rior face; 2 c, transverse section near middle of spine ; 2 d, portion of anterior 

 margin showing arrangement of tubercles. Coal No. 5 ; Carlinville, Illinois. 

 3. Fragment of an undetermined spine, enlarged. Middle Coal Measures ; Dallas county, 

 Iowa. 



