VERTEBRATES. 367 



This tooth illustrates the difficulty of establishing a satisfactory classification 

 among Plagiostomous fishes by a comparison of one set of organs alone ; for, 

 with the general form and structure of Deltodus, it has the crown surface cov- 

 ered with alternate bands of dense and porous tissue, scarcely different from 

 those which have been relied upon as the distinctive character of the genus 

 Pcecilodus. In Deltodus undulatus (vol. 2, p. 98), the crown surface is 

 waved on its lower half, though there is little difference in the porosity of the 

 ridges and furrows, and the triangular, deltoid outline is distinctly marked. 

 In D. cingulatus (vol. 2, p. 99), the bands of dense and porous tissue are 

 much more strongly defined, and that tooth would perhaps naturally fall 

 into Mc'Coy's genus Climakodus (or, as he writes it, Climaxodus), taking the 

 name of C. cingulatus. It is of very little consequence in which of these two 

 none-too-well defined generic groups, Deltodus or Poecilodus, this is placed, 

 but it is of consequence that this so strongly marked and conspicuous fossil of 

 the Keokuk limestone should be made known, that it may be used for geologi- 

 cal purposes. 



Formation and locality : Keokuk limestone; Warsaw, Illinois. 



DELTODUS LITTONI, N, and W. 



PI. iv, fig. 8. 8a. 



TEETH of medium size, one inch and three-quarters long by one 

 inch wide at broader end, very thick and strong, triangular in 

 outline, strongly convolute, narrow end terminating in an acute 

 angle, opposite and broader end regularly arched; crown sur- 

 face mainly occupied by a very strong but obtuse ridge which 

 follows the line of curvature along one of the margins. This 

 ridge is bordered by a broad, shallow furrow which reaches to 

 the oblique lateral border, and which is slightly raised ; entire 

 enamelled surface coarsely granulo-punctate, but otherwise 

 smooth. 



In form and size this tooth approaches that of D. stellatu-s, N. and W. (vol. 2, 

 p. 97), but is more convolute in form, thicker, the crown surface smoother, the 

 rid^e less angular, the punctation simpler. There is no other species for which 

 it is likely to be mistaken. 



Dedicated to Prof. A. Litton, of St. Louis, Mo. 



Formation and locality: Lower Carboniferous limestone ; Boone county, Mo. 



