40 PALAEONTOLOGY OF ILLINOIS. 



eiated with various other modifications of the structure of the fishes to which 

 they belong, by which their whole economy was changed, there can be no doubt. 

 We have, therefore, felt justified in placing them in a generic group by them- 

 selves. 



Figs. 9, 9 a, 9 b and 9 c represent the anterior and posterior surface, side view 

 and profile of a large specimen from the Chester limestone. 



Formation and locality ; Chester limestone, Chester, Illinois. 



Genus PETALORHYNCHUS, Ag. 

 Petalorhynchtjs stkiatus, N. and W. 



PL II, Figs. 8, 8 a, 8 b. 



Teeth small and thin; crown sub-pentagonal iu outline, flat 

 or slightly arched laterally, summit sloping either way to two 

 lateral angles where the tooth is broadest, thence the sides con- 

 verge to the basal folds ; cutting edge sharp, finely and rather 

 irregularly crenulate ; anterior surface slightly less high than 

 the posterior, central portion raised into a broadly rounded 

 vertical ridge, bordered by parallel shallow depressions, upper 

 two-thirds uniformly marked by fine vertical or divergent 

 striae, showing the courses of the calcigerous tubes, which are 

 somewhat branched; basal folds few, (2-3), strong mesial 

 angle turned up instead of down; posterior surface plane, 

 slightly curved backward at the base, smooth below, striated 

 above; imbricated folds at base 2-3, broad, nearly straight, and 

 horizontal; root unknown. 



This unique and pretty species bears a distinct resemblance to P. sagittatus, 

 Ag. {Mc Coy Brit. Palaeoz. Foss. p. 636, PI. 3 i, fig. 2, 3), but may be readily 

 distinguished from it by its thin, flat crown, so distinctly and peculiarly striated; 

 the reversed basal angle of the anterior surface, etc. 



Figures 8, 8 a, 8 6, are views of the front and rear faces of the crown, and 

 section, all natural size. 



Formation and locality: Burlington limestone, Quincy, Illinois. 



