74 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
ments in these four closely related genera, more especially between 
Campodus and Edestus. Starting with Campodus as the least special- 
ized member of the series, 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 the 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 lateral series, 
is obsolescent in the symphysial teeth. A continuous nutritive 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 (ef. this author’s fig. 
39). And, finally, the curvature of the series is not greater than in the 
corresponding arch of Cestracion or other existing sharks having the 
symphysial cartilage well developed (Lamnidz, Carchariidz), 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 (ef. text-figure 6) with serra- 
tion of the apical margins, a pronounced forward 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 more closely crowded 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 progressive stages which 
lead up to this condition are indicated by the three species of Cam- 
pyloprion in the order named, — C. lecontei, C. davisit, 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, 
— 
all 
