4U PALAEONTOLOGY OF ILLINOIS. 



portion of the crown, resemblances with Petalorhynchus, although the 

 relationship is more remote than in the case of Peltorhis ; but in both 

 the root is totally different from that of Petalorhynchus. The genus has 

 two representative species in the Chester division of the Lower Carbon- 

 iferous. 



Fissodtts BiFrDtrs, St. J. and W. 



PI. XIII, Fig. 1, 2. 



Teeth small. Convex crown-face very low, sublimate in general out- 

 line, gently arched laterally and more strongly so along the compressed 

 crest, which is deeply cleft midway, forming two strong, acutely pointed 

 lobes, basal fold indistinct, gently arched downward in the middle and 

 strongly curved downward terminating in the auriculate lateral angles ; 

 concave face gently depressed, faintly swollen above in the coronal 

 cusps and again in the basal region, which is deeply and somewhat 

 angularly arched downward, abruptly defined from the root below, and 

 occupied by a relatively wide coronal belt composed of three to four or 

 five narrow imbrications, which become exceedingly attenuated ascend- 

 ing the diverging lateral margins towards the acutely produced lateral 

 angles of the crown, where they are usually obsolete ; coronal surfaces 

 invested with a smooth polished enamel layer, which on being worn away 

 along the crest exposes to view a vertical striato-punctate structure. 

 Inferior or basal surface of tooth irregularly oval or subcircular in out- 

 line, moderately convex, and more or less uniform with the convexity of 

 the convex crown-face, from which it is faintly denned by a slight sulca- 

 tion extending along and just beneath the very narrow coronal fold, 

 lower surface slightly flattened and suddenly produced into a long, 

 narrow, tapering root, which is flattened in the same plane as the crown, 

 somewhat deeply excavated in the inner face and flanked by rather 

 prominent lateral bosses, which shade into slight lateral ridges along 

 either margin of the root towards the lower extremity, which is slightly 

 rounded or truncated ; basal region and root faintly roughened or quite 

 smooth. Lateral diameter of tooth .29 inch, vertical diameter .38, ele- 

 vation of convex crown-face .OS inch, or about half the hight of the 

 concave face. 



The teeth comprised in the present species are remarkable for their 

 elegant and symmetrical proportions, and the persistency of the dis- 

 tinctive characteristics, as shown by the slight individual variation 

 observed iu a fine suite of specimens. The deeply cleft, sequi-lobed 

 crest readily distinguishes the form from others occurring in the same 

 horizon. 



Position and locality : Not uncommon in the upper and lower ichthyic 

 horizons of the Chester formation; Chester, Illinois. 



