PTYCIIODUN. 



2-43 



a continuous transverse ridge; but in a lower median tooth (fig. 11), evidently 

 of the same set, a few of the regular transverse ridges are again distinct. 



Of the typically marked teeth of P. drear reus there are thus three varieties, 

 namely: (1) diminutive smooth teeth, to be known as var. levin ; (2) depressed 

 teeth with more or less regular transverse ridges, to be named var. depressus ; and 

 (3) depressed teeth with very irregular transverse ridges, to be named var. otveni. 

 It only remains to add that there is still a fourth variety in which the transverse 

 ridges are more or less recurved into loops at the ends. 



Teeth of this form exhibit sometimes a normally raised, sometimes a depressed 



Fig. 77. Ptychodus decurrens, Agassiz ; teeth of lower jaw in original order, nat. size. — Chalk; Kent. 



B. M. no. 40056. 



crown, and there is every gradation between the tapering ends and the complete 

 loops of the transverse ridges. The well-preserved portion of upper dentition 

 shown in PL LI, figs. 4-6, indeed, exhibits teeth with transverse ridges which end 

 in concentric loops at the outer border while tapering in the normal manner at the 

 inner border. In the small median teeth (o') the central area is much raised and 

 the ridges are directly transverse, without curvature at the ends (fig. 5). In the 

 inner paired series (l') the crown of each tooth is considerably raised, and the 

 concentric looping of the ten transverse ridges at the outer border is especially 

 conspicuous, while their tapering inner ends are merely inclined forwards. The 

 granulated marginal area is very narrow, and is almost lacking at the lateral 

 borders of the teeth in the more external rows, which are low and more or less 



