360 PALAEONTOLOGY OF ILLINOIS. 



rapidly narrowed downward where it becomes thinner, and 

 terminates in a sharpish edge ; posterior face of root shorter 

 than anterior, and marked by strong, vertical ridges and fur- 

 rows. 



In general form the teeth of this species resemble those of H. compressm, 

 but are smaller and less flattened. The character by which it may be distin- 

 guished from all other known species of the genus is the rugosity of the crown 

 surface. 



Formation and locality : Coal Measures ; Collinsville, Illinois. 



HELODUS COMPRESSUS, N. and W. 



PI. iii, fig. 15, Ua. 



TOOTH small, much compressed or flattened ; crown yoke- 

 shaped, smooth, coarsely punctate, bearing a flattened, smooth, 

 sub-central, medial cone, arched upward at base; root less in 

 hight than crown, bevelled to an edge below, and marked on 

 either surface with vermicular lines. 



In outline this species resembles H. consolidatus, N. andW. (vol. ii, p. 87,'P1. 

 vi, fig. 2) but is much smaller and more flattened. 



Formation and locality : Burlington limestone; Quincy, Illinois. 



GENUS LOPHODUS, N. and W. 



TEETH of various forms, many transversely elongated ; the 

 crown raised into several summits, and traversed in its longest 

 diameter by a sharp crest ; root vertical, flattened. In others 

 the medial cone is greatly developed, the lateral ones obsolete 

 or represented by buttress-like wings that are given off on one 

 side of the tooth. The medial cone is laterally compressed, 

 and bears a sharp crest along its antero-posterior medial line. 



The most elongated of these teeth, taken by themselves, would be included 

 in the genus Orodus, being formed altogether on the same plan ; but those 

 bearing the high, ancipital, medial cone are very different from any found in 

 the dental series of Orodus, and approach the form of the ancipital teeth of 

 some of the mesozoic reptiles. 



