132 PALAEONTOLOGY OF ILLINOIS. 



species represented by the present form would then have to be re- 

 legated to Chitonodus. 



Geological position and locality: Warsaw limestone ; near the mouth 



of Piasa creek, above Alton, 111. 



POZCILODUS ST. LUDOVICI, St. J. and W. 



PL VIII, Fig. 8, 12. 



Teeth of small size, representing the posterior forms of the upper 

 and lower jaws; dental armament of the anterior portions of the 

 jaws unknown. 



Mandibular posterior teeth strongly arched in the direction of in- 

 rollment, in outline presenting the usual subspatulate form. Antero- 

 lateral border relatively very short, forming nearly a right angle 

 with a line connecting the angles of the inner margin, extremely 

 attenuated and contracted from the anterior coronal ridge ; postero- 

 lateral border converging toward point of inrollment at an angle of 

 50, more -or less, with the opposite border, basal portion deeply 

 channeled and widely expanded beyond the coronal limits posteri- 

 orly, terminating in the acutely rounded spur of the posterior ex- 

 tremity; inner margin making a deep regular concavity in the 

 region of the posterior depression, broadly arched round the base 

 of the median lobe, with a shallow concavity in the intermediate 

 depression in front, and angularly arched round the anterior ridge, 

 beyond which the course is suddenly and very obliquely deflected 

 forward to the obtuse anterior angle. The coronal region is strongly 

 marked by the characteristic contour features. Anterior lobe nar- 

 row, prominent, culminating in a rounded crest moderately oblique 

 to the antero-lateral border, in front of the abrupt slightly convex 

 slope, descending to the coronal limits, where it is bordered by a 

 narrow slightly depressed belt, beyond which the basal portion pro- 

 jects as a wide, rapidly narrowing apron to the anterior extremity 

 of the tooth; posterior slope equally abrupt but of less vertical ex- 

 tent; intermediate depression equaling the breadth of the anterior 

 lobe, in perfect teeth showing a nearly plane area narrowing from 

 within outward, denned from the bordering elevations by a slight 

 angulation on either side, in worn and mature examples the depres- 

 sion presents a more or less deep, regular transverse concavity; 

 median or principal lobe situate a little posterior of the median 

 line, broadly and regularly arched transversely, the abrupt and 

 longer slope descending into the posterior depression which is tra- 



