462 PALEONTOLOGY OF ILLINOIS. 



union with the opposite spine; posterior margin regularly curved with 

 the posterior basal margin, lateral surfaces ridged with three or four 

 low, rounded, enameled longitudinal costae, rarely bifurcated, and spaced 

 by aboutf their own diameter, except in front, where a broad intercostal 

 space intervenes between the first lateral ridge and the broad, rounded 

 ridge of the anterior margin. The posterior face is very indistinctly 

 shown in our specimens, but appears to be of moderate breadth and 

 rounded or keeled; in one specimen from St. Louis, the tubercles, 

 described above as ascending the lower third of the posterior face, 

 appear as high up as the middle, or even higher, where they form a 

 single row of rather widely spaced, laterally compressed, hooked den- 

 ticles, apparently confined to the keel or median line, as shown in figure 

 1 a. The pulp cavity is relatively small, slightly posterior to the centre, 

 but it apparently expands inferiorly, occupying the greater diameter of 

 the basal portion in the condition of a deep channel opening along the 

 posterior borders of the inserted shaft. A specimen of small size affords 

 the following comparative measurements : greatest diameter of base 

 .20 inch, hight to the angle of the anterior and posterior arms .35, 

 greatest diameter of anterior arm .12, diameter near extremity about 

 .05, length in a direct Hue .95; greatest diameter of posterior spine .12 

 inch, length .31; the largest specimens attained at least twice the dimen- 

 sions of that given above. 



The above species is represented by several fragmentary and more or 

 less perfectly preserved specimens in the collection of Mr. Van Hoene 

 and those of the authors, which afford sufficient details for the defini- 

 tion of their distinctive characteristics, but which unfortunately do not 

 exhibit certain features necessary for a perfect understanding of the 

 generic characters, so far as they depend upon external features, and 

 which are not apparent in the original described by McCoy. These 

 cousist in the imperfect condition of the extremity of the singular ante- 

 rior appendage, which recalls in shape and position the bony appendage 

 which arises from the cranial region of the existing Chimcera, and the 

 apparent distortion of the basal shaft and posterior spine, which render 

 it difficult to arrive at a correct understanding of the outline and trans- 

 verse sections, the pulp cavity and the contour of the posterior face. 

 As mentioned above the posterior face of the posterior spine appears 

 to be rounded or moderately keeled, the postero-lateral angles very 

 obtuse, and in worn specimens scarcely observable, but in others appa- 

 rently occupied by an enameled ridge. A single small specimen in the 

 collection of Mr. Van Hoene, however, exhibits the superficial features 

 of the posterior spine in nearly perfect state, showing the delicately 

 ridged postero-lateral angles, and the median keel bearing upon its 

 crest a row of relatively long, compressed, trenchant and slightly 



