]'.\s The American Geologist. September, 1894 
much resembling those which may be seen on the jaws of the 
existing sharks. From the hindmost point and along the 
lower edge runs a strong corrugation, formed rather by a 
folding than by a thickening of the bony plate, and this ex- 
tends forward at least two-thirds of the length of the jaw and 
gradually fades away. Outside of this and on the same inner 
face is a furrow corresponding to a strong ridge on the outer 
side, extending forward in the same manner and to about the 
same extent. 
The upper edge, however, shows the most obvious charac- 
ters. At a point about one-third from the hinder extremity 
commences a strong shelf or corrugation in the bony plate on 
its inner face, which is equally conspicuous on the outer side 
and may be well seen in the two sections given in the plate. 
The outer wall of this shelf has been crushed in to a small ex- 
tent, but not so as in any way to disguise its form. Evidently 
it was the base on which was set the epithelial membrane 
that carried the teeth, and the recess in the jaw above the 
horizontal shelf served the same purpose as the similar recess 
in the jaw of an existing shark, to protect the young teeth 
and to keep them out of the way of the functional ones. The 
close resemblance between the two, Devonian and recent, may 
be seen by looking at the section of the latter given in the 
plate where the teeth are shown in place. 
The symphysial articulation is distinct, about one inch and 
a half in length, straight and retreating below. 
There are in the perpendicular wall of the dentiferous mar- 
gin of the jaw six or seven very slight undulations or recesses 
of the bone which may and probably do mark the position of 
as many series of immature teeth. In that case the teeth must 
have been of large size and distant. Probably they were both. 
The intervals average about one inch each, but the vertical 
wall is rather less than an inch high. The teeth were there- 
fore limited in bight to probably about an inch. 
The reference of this fossil to Cladodus i- of course pro- 
visional in tin- absence of teeth. But its general structure 
establishes its selachian affinity, and its resemblance in gen- 
eral to other fossils from the same stratum warrants a clado- 
dont association, at least for the present. 
The only other fragment which was found in close proxim- 
