VERTEBRATES. 339 



tooth, but the peripheral region and base is left to later processes for 

 completion. It is often of importance in determining the identity of 

 isolated teeth to be able to recognize the comparative degree of devel- 

 opment of the individuals under examination. In the present examples 

 the much-worn anterior teeth, with their-strong and usually entire base, 

 would scarcely be recognized as specifically identical with the immature 

 coronal shell of the incomplete teeth, whose partially formed bases 

 have entirely perished, but whose coronal markings are preserved with 

 extremest sharpness. 



In the group under consideration a combination of characters occurs 

 which seem to indicate affinities with both the Orocli and Chomatodi, as 

 represented by "the genera Mesodmodus and Yenustodus. Yet they may 

 prove to be widely distinct from either of the above groups, and more 

 closely allied to the Cochliodonts. 



The representatives of the group belong to the Lower Carboniferous 

 formations. The Orodus minusculus of Messrs. Newberry and Wor- 

 then, of the Keokuk limestone, is a representative species, and closely 

 allied to the typical form D. tumidus, of the St. Louis limestone. 



Desmiodtjs ttjmidtjs, St. J. and W. 



PI. Xa, Fig. 7-9. 



Teeth minute, varying in outline from subtriangular to long-elliptical, 

 strongly built. Crown moderately arched laterally, obtusely rounded 

 at the extremities, with an obtusely angular crest culminating in a 

 strong, tumid, median prominence, which is nearly central in the late- 

 rally abbreviated varieties, but more or less eccentric in the elongated 

 teeth, lateral portions of the crest bearing three to five, more or less, 

 rather strong denticulations, which diminish in size towards the extrem- 

 ities and similar in shape to the median prominence, posterior face 

 gently arched in both directions, sometimes sigmoidally so, anterior 

 side more abrupt, faintly concave or nearly plane vertically, more or 

 less produced in the median region where it is interrupted by the strong- 

 vertical angulation supporting the median cone, at the base of which, 

 in the angle of the basal fold, occurs a small node-like protuberance ; 

 basal borders of the crown abruptly inbeveled and occupied by a simple 

 wide coronal fold, the sharply defined upper edge of which slightly 

 overlaps the produced borders of the superior coronal region, aud which 

 encircles the crown in a continuous baud, gently arched upward in the 

 median region and at the produced lateral extremities ; coronal surfaces 

 quite smooth and polished, the denticulations in the crest being detiued 

 by faint vertical sulci, and the anterior median buttress distinctly cari- 

 nate. Base equal to the crown iu depth, slightly less iu lateral diatne- 



