138 IOWA GEOLOGICAL SURVEY 



Some of the above-mentioned differences may be due in part 

 to conditions of preservation, but one important characteristic, 

 as appears from the illustration, is found in the absence of a 

 descending spine, or even a blunt projection, at the anterior 

 margin of the lower dental plate. 



Formation and locality. Glen Park limestone (Kinderhook) ; 

 Grlen Park, Jefferson county, Missouri. 



Accompanying Detached Parts, presumably referable to 



Ptyctodonts. 



1. dorsal fin-spines. 



Genus HETERACANTHUS Newberry. 

 (Syn. Gamphacanthus Miller) 



Spines of moderate size, broad and laterally compressed at the 

 base, but soon becoming subtriangular in cross-section, nearly 

 rectilinear, and gradually tapering toward the acute apes. In- 

 ternal cavity relatively large, extending nearly to the apex, and 

 open along the posterior margin for a considerable distance. No 

 anterior keel; posterior denticles not observed. Exserted por- 

 tion ornamented with finely crenulated longitudinal ridges which 

 become smooth and flat with wear, and are separated by fine, 

 denticulate intercostal grooves. 



Heteracanthus politus Newberry. 



1889. Heteracanthus politus J. S. Newberry, Monogr. U. S. Geol. Surv. 16, p. 



66, pi. 21, figs. 4, 5. 

 1892. Gamphacanthus politus S. A. Miller, North Amer. Geol. and Pal. p. 715. 



1898. Heteracanthus politus C. R. Eastman, Amer. Nat. 32, p. 552. 



1899. Heteracanthus politus C. R. Eastman, Journ. Geol. 7, p. 282. 



1907. Heteracanthus politus C. R. Eastman, Mem. N. Y. State Mus. 10, p. 82. 



Spines attaining a total length of about 20 cm, very broad 

 at the base, and with Ctenacanthus-like ornamentation. The lon- 

 gitudinal costae, which are rather numerous and closely crowded, 

 become perfectly smooth in the worn condition, their presence 

 being indicated only by the fine and deep intercostal grooves, — 

 the so-called "sinuous or denticulate longitudinal sutures" of 

 Newberry. More or less variation in size and number of the 

 costae is to be observed among different examples. The basal 



