VERTEBKATES. 39 



This elegant little species resembles, in size and outline, A. minutus, but is 

 readily distinguishable from it by its discernible, though small, root, and th« 

 greater relative height of the anterior face of the crown — differences distinctly 

 shown in the profiles of the two species. 



Figures 7 and 7 a, represent, respectively, the anterior and posterior faces, 

 and the profile, natural size. 



Formation and locality: Burlington limestone, Quincy, Illinois. 



Antliodus robustus, N. and W. 



Plate II, Figs. 9, 9 a, 9 6, 9 c. 



Teeth of medium size, thick and massive, much broader than 

 high; crown twice as broad as high, upper margin regularly 

 arched, somewhat rounded, obtuse and porous ; anterior surface 

 semi-elliptical, half the height of tooth, regularly arched verti- 

 cally and laterally, smooth and polished, in its normal state, 

 except along the upper margin, where it is striated by the ex- 

 posed enamel tubes; below these tubes inosculate, producing a 

 reticulated, still lower a dotted porosity; anterior coronal ridge 

 narrow, bow-shaped, prominent, and showing several folds of 

 enamel; posterior surface smooth, regularly concave in both 

 directions, one-third higher than anterior face, exclusive of the 

 broad coronal ridge, which is arched downward or slightly bow- 

 shaped, showing 5-6 distinct imbricating folds, of which the 

 upper one is much the broadest; base nearly smooth; root two- 

 thirds the entire breadth of the tooth, very short, slightly pro- 

 jecting below the posterior coronal ridge, tubercled or rough- 

 ened, not divided. 



This, and the other species of the short-rooted group of Petalodonts, to which 

 it belongs, exhibit the singular feature of a crown which is essentially that of 

 the typical species of Petalodus, but having the root, so conspicuous in those 

 species, dwarfed to a mere tuberculated ridge, set on the base, as in the present 

 species, A. parvulus, A. cucullus, etc. — or still more rudimentary or wholly 

 wanting, as in A. simplex and A. minutus. 



That this departure from the normal type, in the form of the teeth, was asso- 



