VERTEBRATES. 35 



Genus PETALODUS, Owen. 

 Petalodus destructor, N. and W. 



PI. II, Figs. 1 apd 3. 



Teeth large ; crown sharped, compressed, more or less arched 

 laterally, rhomboidal in form, with curved outlines, somewhat 

 acumenate at apex; cutting edge very sharp, crenate; ante- 

 rior face smooth and polished, broadly rhomboidal, lateral 

 angles very acute, upper one slightly rounded, lower one broadly 

 so; posterior surface one-third to one-half higher than anterior 

 face, smooth and polished, terminating below, like the anterior 

 surface, in a band of 5-6 imbricated folds, which are broader 

 and more strongly marked behind than before; root nearly 

 smooth, broad, and compressed above, narrowed and thickened 

 below, where it is rounded, and deflected forward; posterior 

 face about the height of the posterior face of curve; anterior 

 face one-third higher. 



Entire length of a large specimen, 2 inches, 3 lines ; height 

 of posterior surface of crown, including basal folds, 1 inch, 3 

 lines; of anterior surface, 9 lines; breadth, 1 inch, 9 lines; 

 length of root, anterior surface, 1 inch, 6 lines; of posterior 

 face, 1 inch. 



These large and fine teeth, as will be seen by reference to the figures given, 

 must have formed the dental armament of one of the most formidable of the 

 Carboniferous Placoids, and the equal in 'size and prowess to the gigantic Sau- 

 roids of that age. 



In some specimens, included in the collection, the crown is an inch in height 

 by one and a half inches in breadth — equaling in size the teeth of the largest 

 living species of Carcharias, while the root, comprising half the tooth, buried 

 in the integuments, must have given to the trenchant crown a firmness and 

 efficiency far greater than that of any of the modern shark's teeth, which have 

 only a short tuberous base. 



Some of the specimens show much wear, but still retain the sharpness of 

 cutting edges. This would seem to show that the teeth of the opposite jaws 

 shut together like a pair of shears, and the attrition which they suffered served 

 only to keep them in order. 



