108 PALEONTOLOGY OF ILLINOIS. 



degree of probability as pertaining to the mandible of the species. 

 The latter teeth exhibit in a marked degree homologous relationship 

 to Cochliodus, possessing the same general outline and coronal con- 

 tour distinguishing the large posterior teeth of the mandible of that 

 genus, characterized, however, in accordance with the distinctive 

 peculiarities appertaining to the present genus. The latter form 

 is trapezoidal in outline, and in the arching and inrollinent of 

 the tooth it agrees with the opposed form; posterior wing narrow, 

 basal rim extending conspicuously beyond the limits of the shal- 

 low, rounded, inbeveled enamel fold; median lobe situateda a little 

 posterior of the middle, broadly and regularly arched transverse- 

 ly, culminating in a tumid crest, the slope on the one hand 

 merging into the posterior depression, on the other denned by a 

 slight angulation from the anterior area, which presents a broad 

 plane or gently-convex surface, with faint impressed furrow near 

 the border and nearly equal in breadth to the median lobe ; an- 

 tero-lateral border quite obliquely truncated outward and back- 

 ward from the sub-acute inner angle ; the inner margin is gently 

 arched between the angles, somewhat more strongly arched around 

 the base of the median prominence, and slightly deflected on nearing 

 the posterior extremity. The external enamel coating investing the 

 inner portion of the crown shows distinct lines of growth parallel 

 with the inner margin, the worn surface regularly and minutely 

 punctate, precisely after the manner observed in the opposite teeth 

 of the upper jaw. 



A medium-size posterior tooth of the upper jaw measures, in 

 transverse diameter across the inner margin, 12.5 mm. ; length 

 along antero-lateral border to point of inrollment, 4 mm. ; greatest 

 diameter diagonal to the longitudinal axis, 5.5 mm. A large-sized 

 mandibular posterior tooth measures, across the inner margin, 15.5 

 mm. ; length along antero-lateral border, 7 mm. ; longitudinal di- 

 ameter, at middle 9 mm. 



Mr. Van Home's collection contains a single minute specimen 

 of a tooth which clearly belongs to the form which we have 

 already recognized from the Upper Burlington fish-beds, from ex- 

 amples- in the collection of Mr. Springer, and which is referred 

 to the mandibular medium form of Stenopterodus. The present tooth 

 presents the same resemblances in surface punctation, as compared 

 with the posterior forms of S. parvidus, as in the former instance 

 was noted in connection with this form and S.planus. The latter 

 specimen attains a breadth across the inner margin not exceeding 



