1883.] NATURAL SCIENCES OF PHILADELPHIA, 109 



• 



portion is, on the superior face (if the piece belong to the inferior 

 jaw, and vice versa), trans\ersely ridged and grooved, after the 

 manner of the genus Janassa. 



Char, specif. — The smooth border is wide above and below. 

 Its edge is produced into a median projection, which is decurved. 

 On the inferior surface it is marked by shallow grooves, which 

 radiate from the gi'oove which bounds it posteriorly, extending 

 nearly to the free edge, Posterior to the bounding groove, the 

 surface is smooth. The posterior surface above has its grooves 

 concentric with the curved free margin. The ridges are narrow, 

 and step-like in position, presenting their free edges backwards. 

 There are no grooves other than these steps. They have an 

 angular curve opposite to the angle of the free margin, and at 

 the angle the groove which separates them is narrowed, while it 

 widens at other points. Free edge of border thickened ; surface 

 everywhere smooth. 



Measurements. m. 



Length of fragment transversely, . . . '014 



Length of fragment anteroposteriorly, 

 Width of border area at median suture. 

 Seven cross ridges, . . . 

 Thickness at suture at cross-ridges, 



•Oil 

 •005 

 •005 

 •002 



'Ctenodus heterolophus .=p. nov. 



This species is represented by a single broken tooth, which 

 presents remarkable characters. It had apparently, when perfect, 

 but three crests, which differ greatl}' in length, diminishing very 

 rapidly from the first or marginal crest. 



The crest just mentioned is not only longer, but niuch more 

 elevated than the others, except at the base, where the second 

 crest is the liighest. But while the first rapidly rises, the second 

 retains its elevation, and then descends, forming a convex edge, 

 of which the distal part is obtusely serrate. The proximal part 

 of the first crest is worn by friction with the opposing edge of 

 the opposite jaw into a sharp edge, below which its base is 

 covered by a thin laj'er of the shining cementum which invests 

 the teeth and sides of the second crest. The amount of this 

 shining layer is thus more extensive than in any other species of 

 Ctenodus known to me. The third crest, judging by its base of 

 continuity with the second, is very small. 



