74 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



meuts in these four closely related genera, more especially between 

 Campodus and Edestus. Starting with Campodus as the least special- 

 ized member of the aeries, we find that the symphysial teeth are but 

 little differentiated from those of the lateral series, except that they 

 are greatly 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 symphysial cartilage of tlie jaws without being anteri- 

 orly elongated. The coronal apices are very stout, rather obtuse, and 

 with sharp, non-crenulated cutting edges, although faint wrinkles some- 

 times appear in the youngest-formed^teeth. The longitudinal ridge ex- 

 tending over the coronal surface, so prominent in the latei'al series, 

 is obsolescent in the symphysial teeth. A continuous nuti'itive canal 

 appears to have perforated the series in the median line immediately 

 below the base of the apical portion, or in about the same relative 

 position as shown by Karpinsky in Helicoprion {cf. this author's fig. 

 39). And, finally, the curvature of the series is not greater than in the 

 corresponding arch of Cestracion or otlier existing sharks having tlie 

 symphysial cartilage well developed (L^mtinidie, Carchariidie), nor is 

 there any difference in the number of segments. 



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

 symphysial teeth have become considerably differentiated in form from 

 the lateral series, their chief modifications consisting in a greater com- 

 pression of the crown from side to side (cf. text-figure 6) with serra- 

 tion of the apical margins, a pronounced fcjrward curvature toward the 

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

 sion of gouge-like troughs or sheaths. With increasing compression of 

 the segments, their basal portions become moi'e closely ci'owded to- 

 gether, and more intimately fused at their extremities into a common 

 mass of vasodentine, in consequence whereof spiral enrollment of the 

 series follows almost as a matter of necessity, since the individual seg- 

 ments can no longer be shed with age. In Helicoprion the lateral 

 compression, fusion, multiplication, and spiral enrollment of symphysial 

 teeth is carried to an extreme degree, and the pi'ogressive stages which 

 lead up to this condition are indicated by the three species of Cam- 

 pyloprion in the order named, — C. lecontei, C davisii, and C. annectans. 



Progressive modification takes place in two directions amongst these 

 genera, starting with Campodus. In the three species of Campyloprion 

 just enumerated, and one of Helicoprion, the tendency is toward 

 enlargement of the apical at the expense of the basal portion of the teeth, 



