VERTEBRATES. 305 



collections from that horizon. A few imperfect specimens from the 

 Keokuk limestone, which have come to our notice, are undistinguishable 

 from the teeth in the above mentioned subjacent deposits, yet they may 

 prove to be specifically distinct. The individual variation in size 

 observed in this form has lead us to believe that it does not constitute 

 a part of the dentition of other forms known from the same horizon. 

 They are essentially dissimilar in many of the superficial details from 

 other Chomatodi with which they are associated, and taking into account 

 the variation in size and the slight modifications of relative proportions, 

 we can scarcely fail to recognize in these variations those features com- 

 mon to the more or less variable dentition of a Petalodont species. 



Position and locality: Upper Burlington fish ■ bed ; at the several 

 localities in DesMoiues and Louisa counties, Iowa; Warren county, 

 Blinois. The Keokuk teeth mentioned above occur at Warsaw, Illinois, 

 and Dr. Williams has obtained the same form in the Keokuk limestone 

 ttt Boonville, Missouri. 



LlSGODUS sereatus, St. J. and W. 



PI. X A, Kg. 17-19. 



Teeth exceedingly small and variable in relative proportions. Crown 

 stout, semi-elliptical in outline viewed from above, more or less acumi- 

 nate, sharp-crested, and interrupted by vertical sulci, which produce an 

 irregular serration, which, however, is scarcely perceptible in some of 

 the more acuminate individuals, or limited to one or two faint indenta- 

 tions well down in the lateral edges of the crest ; convex face nearly 

 vertical, slightly arched laterally or nearly straight, basal margin well- 

 defined from the root and marked by a proportionately wide coronal 

 band, consisting of two imbrications, which span the tooth in a direct 

 course, or sometimes gently arched upward in the middle, and again at 

 the extremities; opposite face half as high again as convex face, but 

 in the more strongly acuminate specimens the two faces are nearly 

 equal in elevation, moderately concave vertically, gently and irregularly 

 arched laterally, median region more or less swollen, basal line promi- 

 nently produced beyond the root, arched upward in the middle, and 

 again abruptly curved upward to the extremities, and occupied by three 

 imbricating folds, which form a band but little wider and continuous 

 with that of the opposite face. The crown is covered with a polished 

 enamel layer, the triturating surface being most strongly marked in the 

 concave side, (though in none of the specimens is it as distinct as iu L. 

 curtus,) and giving rise to the vertical striation and striato punctate 

 appearance commonly observed for a greater or less distance below the 

 crest. The base is well-developed, angular in outline, nearly vertical 



