VERTEBRATES. 305 



surface markings intimately agreeing with the preceding variety, and 

 varying in lateral diameter from .15 to .55 inch, (PI. VII, Fig. 2, 3, 4.) 

 Some of the latter may be individuals of the former variety, whose 

 median prominences have been worn down by attrition ; but it seems 

 probable that the majority of them represent the dental armature of 

 the posterior portions of the jaws, and that of the extreme anterior 

 portion of the jaws may be represented by a peculiar form of small, 

 proportionately slender, high-crested, strongly buttressed teeth, which 

 bear some resemblance to the first mentioned variety, as shown in the 

 illustrations, PI. VII, Fig. 1 a, b. 



This interesting form is one of the most numerously represented in 

 the Upper Burlington fish-bed. It is not apparently allied with other 

 species from the same horizon, but a form occurs in the Keokuk lime- 

 stone, originally described under the name of Helodus gibbosus, 1ST. and 

 "W., (111. Rep. II, p. 79, PI. V, Fig. 3,) which bears most intimate rela- 

 tionship with the present teeth. Fortunately the collection of Mr. Van 

 Hoene affords a fine large example from Hamilton, Illinois, which, 

 though slightly worn, exhibits the specific identity as also the true 

 generic relationship of the original specimen above referred to, showing 

 the latter to be a very worn tooth. Our collections from the same 

 vicinity and from Keokuk and Bentonsport, Iowa, furnish several 

 additional smaller specimens of the same form, so that now we have 

 the means to institute a very satisfactory comparison between them 

 and the form described above. The Keokuk form, while it exhibits 

 about the same variability, is chiefly recognizable by its generally more 

 robust proportions, thicker and more massive base, generally less atten- 

 uated lateral extremities, less sharp crested vertical ridges and coarser 

 punctation of the coronal surfaces. 



There are few species, the dental remains of which are so variable, 

 that possess peculiarities which are so readily recognizable as those 

 which facilitate the identification of the isolated teeth of the present 

 form. Some of its variations are at first sight scarcely recognizable, 

 and especially is this true of the small posterior (?) teeth, the depressed 

 contour and broad, subcircular outline of which widely contrast with 

 the large laterally elongated teeth. But in all varieties the same style 

 of coronal ornamentation and form of base prevails, and a careful 

 scrutiny cannot fail to reveal the same pattern of form, though modi- 

 fied to a greater or less extent, according to the place the teeth held 

 upon the jaw. 



Position and locality : This form is apparently confined to the fish- 

 bed stratum in the Upper Burlington limestone, and its geographical 

 distribution is probably as extensive as that of any other species of the 



