400 PALEONTOLOGY OF ILLINOIS. 



in the anterior shoulder, extremity thin and rounded, gently and regu- 

 larly arched with the posterior margin of the body, line of insertion 

 distinctly marked by slight compression, and indicating a posterior 

 inclination of the spine equal to 45° to 50°. Pulp-cavity oblong iu 

 transverse section, thickness about half the width, occupying the post- 

 erior half of the body, and concealed for two-thirds or more its length 

 from the apex, below the opening of which it forms a deep groove gradu- 

 ally becoming shallower as it approaches the extremity of the broadly 

 rounded base. The surface ornamentation above the basal portion con- 

 sists of nearly round or oblong, obtuse and apparently smooth tubercles, 

 slightly constricted at base, arrauged in longitudinal rows, also with 

 obscure or irregular oblique rows ascending from the anterior to the post- 

 erior margin; the rounded anterior margin is occupied by a single row 

 of large transverse tubercles, flanked on either side by rows of gradu- 

 ally diminishing tubercles, of which rows there are about fifteen in the 

 middle of the spine on either side, flanked by a row along the postero- 

 lateral angles, which latter have their apices silghtly turned downward 

 though not hooked ; the vertical spaces between the tubercles are rela- 

 tively greater in the posterior rows than in the anterior ones, while the 

 rows themselves are closely crowded ; the anterior tubercles are five 

 times more bulky than those of the posterior rows. The basal portion 

 is faintly and irregularly striated vertically. Length of spine about 

 four inches, breadth half an inch, thickness about .16 inch. 



The above description is based upon a nearly entire ichthyodorulite, 

 which presents characters by which we are led to infer its specific dis- 

 tinctness from allied forms described in the present and preceding 

 Reports. Towards the base the tuberculose ornamentation has been 

 worn off, as also patches in the body of the spine ; but their bases are 

 still quite distinctly shown, exhibiting their arrangement in close longi- 

 tudinal rows. It is closely allied to X. anceps (N. and W. sp.) from the 

 horizon of coal No. 8, from which it differs in being proportionately 

 thieker, less compressed along the anterior edge, and the nonstellate 

 or smooth tubercles, of which the larger ones forming the anterior row 

 are more transverse than is the case in the above form. Prom X. mira- 

 Mlis, with which it agrees in general proportions, it may be distinguished 

 by the character of the tubercles, w r liich neither possess the transverse 

 carina or crest nor the stellate ornamentation characteristic of the latter 

 form, besides the absence of the double row of large tubercles which 

 occupy the anterior border in that form ; it is also apparently less erect, 

 the dorsal line indicating a greater backward inclination of the spine. In 

 the present form the tubercles are apparently destitute of any sculptur- 

 ing, low and rounded, with the highest point or obscure apex eccentric 

 or slightly below the middle, which feature is perhaps more apparent 



