ON THE NATURE OF EDESTUS AND RELATED FORMS. 287 



terior and posterior margins, and closely apposed to one another. Lateral series 

 and other parts of the skeleton unknown. Permo-Carboniferous. Type, H. bes- 

 sonowi Karpinsky. 



A comparison of the individual segments in these closely related genera is in- 

 structive, more especially so between Edestus and Campodus. Reckoning the latter 

 as the least specialized member of the series, we find that the symphysial teeth are 

 but little differentiated from the lateral, except that they are excessively enlarged. 

 They are only moderately compressed from side to side, the lateral extensions of 

 their crowns are directed simply forward, without appreciable curvature toward the 

 base, and their fused roots are supported by the Meckelian cartilage without being 

 anteriorly produced. The coronal apices are very stout, rather obtuse, and with 

 sharp, non-crenulated cutting edges, although faint wrinkles sometimes appear in 

 the youngest-formed teeth. The transverse ridge extending over the coronal sur- 

 face, and so prominent in the lateral series slightly ectad of the median line of the 

 teeth, is obsolescent in the symphysial series. And finally, the series is not more 

 strongly arched than is the case in Cestracion or other recent sharks having the sym- 

 physial cartilage well developed; nor does it comprise a larger number of segments, 

 Campodus having at least thirteen, and Cestracion sometimes as many as fifteen. 



In the evolution of Edestus and the more strongly coiled genera, the symphysial 

 teeth have become considerably differentiated in form from those of the lateral series, 

 their chief modifications consisting in a greater compression of the crown from side 

 to side with serration of its apical margins, a pronounced forward curvature toward 

 the base, and, in Edestus, an extreme elongation of the latter into a succession 

 of gouge-like troughs or sheaths. With increasing compression of the segments, 

 their basal portions become more closely crowded together, and more intimately 

 fused at their extremities into a common mass of vasodentine, in consequence whereof 

 a spiral enrollment of the series follows as a matter of course, since the individual 

 segments can no longer be shed during growth. In Helicoprion the lateral compres- 

 sion, fusion, multiplication, and involution of symphysial teeth is carried to an ex- 

 treme degree, the progressive stages which lead up to this condition being indicated 

 by the species of Campyloprion. 



Progressive modification takes place in two directions amongst these genera, 

 starting with Campodus. In Campyloprion and Helicoprion, the tendency is toward 

 enlargement of the apical at the expense of the basal portion of the teeth, with in- 

 crease in the number of segments. A divergent series, however, is represented by 

 the species of Edestus, in which the relatively few segments are not very intimately 

 fused, while their coronal portions become reduced pari passu with the enormous 



