346 PALAEONTOLOGY OF ILLINOIS. 



of as many as five folds, but which are generally less regular than in 

 the large teeth, and which by frequent anastomosis present a net-work 

 of raised lines. In many examples the upper fold is similarly inter- 

 rupted by brief oblique plicre as described above ; the folds of the oppo- 

 site face are less numerous, formiug a narrow belt, often interrupted, or 

 obsolete in the greater portion of the median region ; the denticulated 

 character of the crest also varies widely, the undulations generally 

 obscure and irregularly developed, or obsolete. The base is proportion- 

 ately massive, in form like the preceding variety. 



The varieties above noticed seem to be specifically identical, as deter- 

 mined by the community of their characters, while at the same time 

 they diverge from those noticed in the following description in the same 

 manner and degree. There appear to be no indications of gradation of 

 characters sufficient to account for the peculiarities distinguishing these 

 forms, yet it is possible they may prove to be only varietal expressions 

 of one and the same species. 



The small linear teeth bear, in some respects, striking resemblance to 

 the group of teeth represented by Helodus angularis, ~S. and W., especially 

 in the uniform convexity of the one face and the relatively strong base. 

 The imbricated belt encircling the crown, however, offers a contrast 

 which readily distinguishes the present teeth from that above referred 

 to, and associates them with the large acuminate teeth first noticed. 

 In the marked convexity of the one side, the large teeth exhibit a some- 

 what intimate relationship with Y. Leidyi ; but the large size, central 

 position of the crest, and more simple disposition of coronal folds, con- 

 stitute features quite in contrast with those presented by the St. Louis 

 species. 



Position and locality : Common in the Upper Burlington fish-bed ; 

 Louisa and DesMoines counties, Iowa. 



Venustodus variabilis, St. J. and W. 



PI. IX, Fig. .7-14. 



The collections from the Upper Burlington, especially that of Mr. 

 Springer, contain fine and beautifully preserved series of a form which 

 is so intimately related to Y. tenuicristatus of the Keokuk, that, notwith- 

 standing the somewhat striking contrast presented by extreme examples, 

 it is extremely difficult to define them from the Keokuk form. Indeed, 

 in many worn specimens, there is no distinguishable difference by which 

 the teeth from different horizons might be recognized, and it seems 

 hardly possible to foretell the results of an examination of a larger 

 suite of the Keokuk specimens. The Burlington teeth, however, may 



