YERTEBBATES. 450 



distance to the apex, in which region the tubercles are also relatively 

 larger. The transverse crest of the larger tubercles, especially those in 

 front, often presents an exceedingly sharp carina, and though generally 

 smooth, sometimes the basal portion is marked by short carinas ; 

 throughout the tubercles present a polished enameled surface, and are 

 delicately ornamented with sharp ridges arranged in irregular radiating 

 manner, sometimes reaching the summit, but generally confined to the 

 borders, the upper slope being quite smooth. Where the tubercles have 

 been broken off, leaving only the cicatrices of their bases, the surface 

 presents a longitudinally fluted appearance, the shallow channels or 

 interspaces being fine striato-punctate similar to the smooth posterior 

 face. 



The remarkable ichthyodorulite herein described was discovered by 

 Mr. John Wolf, to whom we are indebted for the use of the specimen. 

 It represents a spine the exposed part of which was probably ten to 

 eleven inches in hight, two or three inches of the upper extremity having 

 been lost, while the entire base, or that portion which was imbedded in 

 the body of the fish, is broken away, only a faint trace of the smooth, 

 area which marked the junction of the dorsal line being visible in the 

 middle angle of the broken spine The body of the spine in the region of 

 the pulp-cavity is crushed so as to bring the opposite walls of the cavity 

 in close contact, causing the distortion shown in the illustration, a,nd 

 which is the condition of nearly all the Xystr acanthi. The anterior half 

 retains its form, enabling the accurate restoration of the posterior por- 

 tion. It is evident also that the opening of the cavity was situated 

 very low, if at all above the dorsal line. 



From the forms described from the Lower Carboniferous formations, 

 under the term Drepanacanthus, the present species differ so markedly 

 as scarcely to require detailed comparison ; while its relation to other 

 forms is noticed in the following description under X. acinaciformis. 



Position and locality : In the carbonaceous shales overlaying coal No. 

 4 or 5, Coal Measures ; near Canton, Fulton county, Illinois. 



Xystracanthtts aclnaciformis, St. J. and W. 



PI. 90, Fig. 2. 



Spine gently arched forward, gradually tapering to the extremity, 

 laterally compressed to a nearly uniform thickness which is about one- 

 fifth the greatest width, obtusely rounded in front, posterior face cari- 

 natc with a slight channel on either side of the keel, posterolateral 

 angles sharply defined and forming a thin edge below the opening of the 

 medullary cavity. Base more compressed, terminating in a sharp angle 



