ON JANASSA BITUMINOSA. 845 



ridges, imbricated forwards, and irregularly undulated, notclied, 

 and tuberculated, and arched forwards at the sides. 



The scoop-like cutting-margin and the crushing-disk we shall 

 call the upper surface, these being, in fact, the only exposed 

 portions, though in reality they represent the surface that is 

 usually considered the back of the tooth. The other or opposed 

 surface, which in ordinary cases would be called the front, we 

 shall name the under surface, because it is undermost as the 

 tooth rests on the jaw. The under surface, then, presents a 

 very peculiar appearance : it is divided into three sharply de- 

 fined, longitudinal, flattened areas or facets, so that in transverse 

 section this side would show as half a hexagon. The central 

 area, which is divided from the two lateral areas by a ridge or 

 angle, is usually a little channelled. The back of the scoop-like 

 cutting-margin is also a little flattened at the sides and centre. 



The root is a depressed process, longitudinally striated, some- 

 what narrower than the crown, and about half its length : it 

 originates in the under surface near to the posterior extremity, 

 and arches backwards and downwards. It is consequently an 

 extension, as it were, of the crown in a plane below the crush- 

 ing-disk. 



When seen in profile the primary teeth are observed to assume 

 a decided sigmoid curve, the anterior scoop-like cutting-margin 

 being turned rather abruptly in one direction, and the posterior 

 extremity of the crushing-disk and root in the other or opposite 

 direction (fig, 4). 



The large primary teeth, which hold a central position, are 

 symmetrical ; the smaller lateral ones, though they agree in 

 every other respect with the above, are more or less oblique ; 

 the sides being unequal, particularly the scoop-like portion, one 

 side of which is more developed than the other. And the root 

 likewise is turned a little to one side, especially in the second 

 lateral. 



The secondary or petalodontoid teeth are not more than |-ths 

 of an inch long, and about the same wide ; they are depressed, 

 and partake otherwise of the general characters of the primary 



