VERTEBRATES. 283 



The present form appears to be most nearly allied to L. costatus of 

 the Upper Burlington limestone, from which it is distinguishable by its 

 greater lateral diameter of base and more compressed and centrally 

 situated coronal cusp. The unique example described belongs to Mr. 

 Van Horne's collection. 



Position and locality : Upper beds of the St. Louis limestone, Alton, 

 Illinois. 



L atv tbdodus hamulus, St. J. and W. 



PI. V, Fig 26. 



Teeth small. Base strong, thick, laterally compressed, posteriorly 

 produced and truncated or rounded, inferior surface nearly plane or 

 slightly arched laterally and nearly straight from front backwards, 

 contracted anteriorly where it terminates in an obtuse marginal promi- 

 nence or border, which distinctly defines the base from the crown, 

 postero-superior surface moderately convex laterally, with a more or 

 less well-defined mesial riilge extending from the base of the cone to 

 the posterior border, both surfaces more or less roughened and longitu- 

 dinally striated. Coronal cusp strong, gradually tapering, prominently 

 produced in front, recurved, laterally compressed, thickest in front, 

 presenting an ovate transverse section, apparently smooth. Hight of 

 tooth about .IS inch, anteroposterior diameter of base .25, lateral 

 diameter .16, vertical depth of base in middle .09, greatest diameter of 

 cone at base .10, lateral diameter .08, hight of cone .15 inch. 



The above form is clearly related to L. calceolus var. robustus, of the 

 Keokuk limestone ; indeed so intimate is the resemblance, that, were the 

 present tooth obtained from the same horizon, one would scarcely hesi- 

 tate, in the absence of means for more critical comparison, to refer it to 

 that form. But, occurring in a position several hundred feet above that 

 in which the above mentioned form is found, in a division which ia 

 otherwise quite distinct from the Keokuk beds, there are also certain 

 peculiarities presented by the form here described, which, should they 

 be found to be persistent, must establish its distinct specific character. 

 The most striking of these apparently distinctive characteristics are, 

 the more prominent forward projection of the cone, and the reversed 

 position of the ovate transverse section, which in the Keokuk form is 

 narrowest in front, and its base less symmetrical. 



Position and locality : Rare in the lower fish-bed of the Chester 

 limestone, Chester, Illinois. 



