108 PALEONTOLOGY OF ILLINOIS. 



roughened by the uniform and relatively coarse punctation ; 

 base relatively thin, of the same form as the crown, but some- 

 what smaller; sides forming remarkably sharp angles with the 

 crown. 



This is a thin and small species, more exact and angular in form than any 

 other hitherto described. The crown is bordered throughout nearly its entire 

 outline by a salient, acute angle, which it forms with the relatively smooth sides; 

 this angle is made more prominent by a narrow and shallow sulcus. The base 

 is very thin, giving to the tooth a less relative thickness than in any other 

 species known, (see fig. 2 b.~) The pores of the surface are relatively large, 

 but circular, distinct and never confluent, as in P. rugosus. 



Figure 2, 2 a, 2 b, represent the top, side and profile, natural size. 



Formation and locality: Chester limestone, Chester, Illinois. 



PSAMMODUS KUGOSUS, Ag. 

 PI. XI, Fig. 3, 3 a. 



Tpeth oblong or quadrate in form; surface of crown plain, 

 slightly arched or concave, according to the place of the tooth 

 in the mouth ; uniformly roughened by relatively large and 

 deep pores, which are frequently confluent, giving it a rugose 

 character ; base, coarse and osseous, of the same form as the 

 crown. 



The specimens in the collection which we have supposed might represent 

 this species, are too imperfect for satisfactory determination. They are, how- 

 ever, readily distinguishable from the finely punctate teeth, we "have referred 

 with doubt to P. porosus, the surface being coarser and rougher, as in P. rugo- 

 sus. This difference, taken in connection with the fact that they are derived 

 from formations which have almost no other fossils in common, has led us to 

 regard them as distinct. 



Prof. McCoy regards both of Agassiz's species as mere varieties, and it is 

 quite possible they are so, but in the teeth before us, the differences of micro- 

 scopic structure are so marked and radical, that conclusive evidence must be 

 required before they will be regarded as specifically identical. 



Figures 3 and 3 a, are top and side views, natural size. 



Formation and locality: St. Louis limestone, Alton, Illinois. 



