GENUS PLATYSOMUS, Agassiz. 

 PLATYSOMUS CIRCULARIS, N. and W. 



PL iv, fig. 2. 



FISH small, two inches long, nearly orbicular in outline, head 

 elongated, acute, granulated, nearly as long as body. Tail very 

 heterocercal, with thirty or more rays; vertebral column pro- 

 longed to the extremity of upper lobe; lower lobe strongly 

 marked, dorsal and anal fins opposite, set far back, broad, ex- 

 tending nearly to caudal fin, each highest anteriorly, anal with 

 about thirty, dorsal with forty rays; rays supported by strong 

 interspinous bones. Scales oblong in outline, smooth, those 

 on the sides three to six times as high as long. 



The discovery of this little fish in the Coal Measures of Illinois, is a fact of 

 great geological interest, as the genus has not before been found in America. 

 In the old world, most of the species are found in the Permian, but a number 

 have also been taken from the Coal Measures near Leeds, England. 



From these, the species before us is apparently distinguished by its small 

 size, more orbicular form, and broader dorsal and anal fins. . : 



Formation and locality : Nodules of iron ore ; Mazon creek, Grundy county, 

 Illinois. 



GENUS PAL^ONISCUS, DeBlainv. 

 PAL^EONISCUS GRACILIS, N. and W. 



PL iii, fig. 4. 



FISH of very small size; body cylindrical and slender; head 

 one-fifth of the entire length, rounded and obtuse anteriorly ; 

 mandibles and maxillaries ornamented with fine raised lines; 

 cranial bones tuberculated (?) ; dorsal and anal fins placed far 



