406 PALAEONTOLOGY OF ILLINOIS. 



cess, gently arched laterally and less than half the vertical extent of 

 the opposite face, surfaces more or less roughened by irregular vertical 

 strire. A medium sized specimen measures in vertical diameter .03 inch, 

 lateral diameter of crown .37, or but little greater than the diameter 

 across the angles of the base, elevation of concave crown-face .45, hight 

 of convex face .30 inch. 



The fine series of the present form which we have had for examina- 

 tion exhibits considerable individual variability, which is in part attri- 

 butable to attrition, probably also to the position the teeth respectively 

 occupied upon the jaws. Probably the majority of the specimens were 

 ■worn down while in use, since the crest and convex face bear every 

 evidence of abrasion in the exfoliation of the enamel layer, and the con- 

 sequent exposure of the minute tubular structure, to which is due the 

 striated appearance of specimens in this state of preservation. In the 

 outline of the crest there is also considerable variation — many speci- 

 mens being prominently acuminate, while others are broadly rounded 

 between the coronal angles : indeed, the rounded outline would appear 

 to be the actual condition of perfect individuals, as indicated by exam- 

 ples in which the enamel-like layer remains, although the same condi- 

 tion of surface obtains in acuminate specimens, and, vice versa, rounded 

 examples present unmistakable evidence of attrition. A large specimen 

 in the collection of Mr. Vajnt Hokne exhibits an anomalous feature, the 

 base being deeply cleft producing a pair of radicles. 



In the present form we have another example of the intimate specific 

 relations existing between many of the American Carboniferous fishes 

 and those of the same formations in Europe. In the present instance 

 we observe the closest resemblance with a form in the Carbouiferous 

 limestone of Armagh, the P. sagittatus, Agassiz. The present form, 

 however, is distinguished by its shorter base, less rapidly converging- 

 outline of the crest, and the more numerous imbrications of the coronal 

 belt — characters which equally serve to distinguish it from P. striatus, 

 T$, and W., of the Upper Burlington limestone. 



Position and locality : Upper beds of the St. Louis limestone ; Alton, 

 Illinois; Pella, Iowa; St. Louis, Missouri. 



Petalorhtnchxjs distortus, St. J. and W. 



PI. XII, Fig. 7. 8. 



In the collections from the St. Louis formations occur a form repre- 

 sented by very small teeth, which we have failed satisfactorily to identify 

 with either of the two species known from the same horizon, although 

 we have not overlooked analogous resemblances, but these are appa- 

 rently irreconcilable with the characters which especially distinguish 



