MUSEUM OF COMPARATIVE ZOOLOGY. 31 
come from an intermediate between the Hybodonti and the Cladodonti, 
a branch from the latter. From the Cladodonti we have the Notidani 
and a portion of the Lamnz. Probably the line leading to the Notida- 
nidz had separated from that of Cladodus before the Carboniferous ; 
but that it belongs to that line appears probable, in view of the affini- 
ties to Chlamydoselachus, and the presence in the upper jaws of Cla- 
dodont teeth and intermediates between them and the most modified 
ones in the lower jaws. 
A great difference between the teeth of Cladodonts and others is to 
be seen in the bases. In Cladodus the teeth are braced posteriorly, by 
a backward prolongation of the base, most often extended under the fol- 
lowing tooth in the same row. As soon as teeth of this character emerge 
from the thecal fold under which they are formed, they come into func- 
tion, on the inner side of the jaws, and continue in use even after passing 
to the outer side. 
Another plan is to be seen in many of the blade- or chisel-like cutting 
teeth, Scymnus and others, or in the broad teeth of the Notidanide. 
Here the bracing is done by a forward prolongation of the base, the 
teeth coming into function only after the extension has passed to the 
outer side of the jaws. In Pristicladodus of McCoy (P. dentatus), and 
its possible descendants, Carcharodon and Carcharias, or in Lamna, there 
is also a slight backward extension, which has the effect of bringing the 
tooth into function sooner by raising the apices of the cusps. 
In Hybodus proper, the teeth are intermediates in character of base, 
being braced neither forward nor backward, or but slightly in either or 
both directions. Such teeth come into use somewhat early, and become 
useless soon after turning to the outer side of the jaw. Between the 
Cladodonts and the Hybodonts there are many intermediates, some of 
which are evidently out of place as now situated in Hybodus. 
Before the discovery of Chlamydoselachus it was supposed the line of 
the Cladodonti had become extinct in the carboniferous. Now, with 
Cladodus at the farther end, their line is extended from the Subcarbon- 
iferous to the present; or, taking Pternodus (Pristicladodus St. John 
and Worthen, not of McCoy, type P. springeri St. J. & W.) —a genus 
allied to both of the preceding genera—as our point of departure, it 
extends from the Middle Devonian. 
How much we are justified in treating Chlamydoselachus as a per- 
sistent type, paleontological research will ultimately determine. The 
reasons for so doing are found in the identity of dental characteristics 
of Cladodus and its recent representative, and in the possession of char- 
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