138 PALAEONTOLOGY OF ILLINOIS. 



PCECILODUS SPEINGERI, St. J. and W. 



PI. VIII, Fig. 19. 



The present species is made known from a few examples repre- 

 senting the posterior or terminal form of the mandible. The teeth 

 attain medium size, strongly arched in the direction of inrollment. 

 Antero-lateral border slightly oblique, outward and forward, to the 

 general course of the inner margin, relatively short, basal border 

 channeled; postero-lateral border converges towards outer inrolled 

 margin at an angle of 50 more or less, with opposite border, basal 

 portion unknown; inner margin strongly deflected backward from 

 the very obtuse anterior angle, making an angle at the base of the 

 anterior lobe, with a regular concavity in the region of the median 

 depression, as also in passing the posterior depression into the 

 acutely rounded posterior extremity, broadly arched round the base 

 of the posterior or principal lobe. The characteristic contour features 

 strongly marked in the coronal region; anterior lobe rising into a 

 narrow sharply rounded crest, anterior slope slightly concave and 

 denned by a narrow rounded fold inbeveled to the inferior or basal 

 border; median depression deeply excavated and regularly concave 

 transversely, marked by a few, three more or less, parallel longitu- 

 dinal folds ; posterior ridge more broadly arched transversely, prom- 

 inent, abruptly declining into the deep posterior depression, which is 

 bordered by the strongly upraised, broad alation. Surface, aside 

 from the already mentioned longitudinal folds occupying the bottom 

 of the median depression, smooth, and minutely and densely punc- 

 tate, the punctse exhibiting a tendency to transverse disposition in 

 lines parallel with the inner margin. Breadth between angles of 

 inner margin 21 mm. ; length along antero-lateral border 5 mm., 

 or a trifle less than half the longitudinal diameter along the anterior 

 lobe. 



The above species, the representatives of which were discovered 

 by Frank Springer, Esq., in whose honor the specific designation is 

 given, offers resemblances almost equally intimate with the species 

 described respectively from the St. Louis and Chester formations of 

 the Upper Mississippi region, so far at least as may be judged by 

 the comparison of the single representative form of the present 

 species. The extreme obliquity or rather posteriorly produced inner 

 margin in front of the anterior lobe, however, distinguished the form 



