406 THE AMERICAN NATURALIST. [Vor. XXXIX. 
As has just been observed, a key to the understanding of 
Edestus-like forms is furnished by the symphysial dentition of 
Campodus, and there lies at hand a simple and reliable test for 
demonstrating their common plan of structure. For those who 
have not actual specimens at command, recourse must be had 
to plaster casts of Campodus and Edestus, which fortunately 
are not rare among the larger museums of this country. If 
one will place side by side the symphysial segments of Campo- 
dus variabilis and Edestus heinrichi, orienting them in natural 
position with the anterior end foremost, one will be struck by 
their almost perfect correspondence, part for part, and line for 
line. > 
First and most conspicuously, it will be noted that the:coronal 
apices of Campodus and Edestus are similarly formed, their 
edges being denticulated and sides. striated. in: corresponding 
manner. It will be seen further that the basal portion of the 
crown projects forward characteristically in both forms, and that 
the different segments overlap and are fused with one another in 
an identical fashion. Only in Edestus is the peculiar trough-like 
base much produced forwardly, and being: composed of vaso- 
dentine, is usually well preserved, whereas in Campodus the 
basal support for the teeth is cartilaginous, and hence unsuited 
for preservation. But the structural resemblance of all parts 
IS SO Obvious, especially when one occupies himself with original 
specimens, that the idea of a homology existing between them 
cannot be avoided. In fact, the evidence appears conclusive 
that Campodus, Edestus, and Helicoprion represent successive 
stages of modification amongst Cestraciont sharks. The impor-- 
tance of these forms from a morphological standpoint is such, 
and the discussion of them so widespread, that it seems desirable 
ioa up an index to their special literature, which is given 
elow. 
