YEETEBBATES. 345 



erior diameter less than that of the crown, with which it is nearly 

 vertical. 



The above group of teeth includes a form originally described by Dr. 

 Leedy under the name Ghomatodus renustus, which may be regarded 

 as a typical example of the genus. Besides the above, two or three 

 additional forms from the superimposed and subjacent formations have 

 come to our notice, which together form a remarkably well-defined 

 generic combination. Each of these forms presents in kind and degree 

 about the same individual variability, conclusively indicating their inti- 

 mate relationship, while, at the same time, each retains its own well- 

 marked characteristics by which it is readily distinguishable from its 

 congeners. 



Ghomatodus (Helodus) denticulatus, McCoy, (Brit. Palre. Foss., p. 618, 

 PI. 3 k, fig. 9 a, b,) of the Carboniferous limestone of Armagh, may 

 possibly prove to be congeneric with the above. The genus is thus far 

 known only from the Lower Carboniferous formations, from which the 

 forms noticed in the following descriptions have been recognized. 



Venustodus robustus, St. J. and W. 



PI. IX, Fig. 15-13. 



Teeth having the same general form as the following described forms, 

 and presenting two well marked varieties, especially distinguished by 

 the angular extremities and vertically arched convex face of the crown. 

 The first or larger variety is distinguished by the gently arched outline 

 and the somewhat sudden culmination of the crest in a strong, obtusely- 

 pointed median cone, which is often truncated from wear ; crest well- 

 defined, nearly central or nearest the convex border, lateral portions low 

 or moderately elevated, and more or less distinctly denticulated ; 

 coronal folds forming a relatively narrow belt in the convex face, some- 

 times obsolete in the region of the swollen median prominence, the 

 upper imbrications often broken into short, oblique plicae terminating in 

 the smooth area, lower folds forming a continuous belt with the broad, 

 well-marked baud of the concave face, where it is composed of four to 

 six folds, which are usually quite regular, or bifurcated at intervals. 

 Base low, narrow, deepest beneath the convex border, inferior surface 

 plain and defined in the angular inner and outer margins. Of the 

 second variety numerous representatives occur in the collection, distin- 

 guished by their smaller size, linear or gradually tapering outline, and 

 low, uniform crest-line which generally traverses the crown centrally, 

 giving to the gently convex face a relatively greater depth than observed 

 in the preceding variety ; the concave face forms a narrow, shallow 

 depression, limited below by the broad coronal belt, which is composed 



