120 PALAEONTOLOGY OF ILLINOIS. 



antero-lateral border to point of inrollment a trifle less than 5 mm., 

 in outline showing the usual quadrilateral figure. The antero-lat- 

 eral border is very oblique in its course forward and backward, 

 coronal enamel forming a strong, rounded fold inbeveled to the 

 channeled basal portion ; inner margin gently arched round the base 

 of the anterior coronal prominence with a slight concavity forward, 

 and a broader one in the region of the median depression ; postero- 

 lateral border unknown. Anterior portion of the crown occupied by 

 a pair of low, regularly transversely arched ridges, separated by a 

 shallow, well-defined furrow, and posteriorly defined by a broader 

 median depression intervening between the anterior and posterior 

 lobes. Surface more or less regularly marked by strong, closely 

 arranged transverse imbrications parallel with the inner margin, 

 and apparently extending over the entire coronal region, besides 

 showing a relatively coarse, crowded punctate structure. 



The species indicated by the fragment of tooth above described 

 apparently constitutes a typical representative of the genus Chiton- 

 odus, of which it is the latest that has, as yet, been made 

 known. It exhibits intimate relations with the Keokuk C. rugosus, 

 but is distinguished by its small size, the greater outward and 

 backward obliquity of the antero-lateral border, and the double- 

 lobed broad anterior prominence ; the transverse imbricated orna- 

 mentation markedly partakes of the generic characteristics associated 

 with species of the genus. 



Geological position and locality; St. Louis limestone ; Alton, Illi- 

 nois. 



GENUS COCIIILIODUS, Agnssiz. 



COCHLIODUS VAN HOENII, St. J. and W. 



PI. VII, 1-10. 



Teeth below medium size, representatives of the mandibular 

 posterior and median, and maxillary posterior forms are known. 



Mandibular posterior teeth in the outline and contour of the 

 triturating surface presenting in a remarkable degree the typical 

 characteristics of the genus, strongly inrolled at the outer margin. 

 Antero-lateral border with a slightly curved outline, the enamel 

 fold occupying about one-third of the vertical space and sharply 



