No. 3.— Some Carboniferous Cestraciont and Acanthodian 
Sharks. By C. R. EASTMAN. 
CoINCIDENT with the marked increase of Pelmatozoa and certain fam- 
ilies of Brachiopods during the Lower Carboniferous all over the world, 
. a race of sharks armed with crushing teeth suddenly acquired dominance, 
became exceedingly diversified, and finally all but passed away towards 
the close of the Paleozoic. Of the very extensive group represented by 
the Cochliodontidz and Cestraciontide, which is at least as ancient as 
the Devonian, only one genus, the so-called Port Jackson shark, survives 
at the present day. With this all of the fossil forms agree in having a 
similar but more or less specialized dentition, in consequence of which 
this creature stands in the same relation to the host of Carboniferous 
sharks with crushing teeth that Nautilus does to fossil Cephalopods. 
Some interesting specimens from the Carboniferous described in the 
present paper throw new light on the structure and relations of Campo- 
dus and the series of Edestus-like forms, all of which are to be regarded 
as members of the Cestraciontide. Spines belonging to the first and 
second dorsal fins of Ctenacanthus, from the Kinderhook limestone of 
the Mississippi Valley, and new species of Acanthodes from the Coal 
Measures of Mazon. Creek, Illinois, are here illustrated and described 
for the first time. A list is also given of the fossil vertebrate fauna 
known to occur at the Mazon Creek locality, including some spe- 
cies not previously reported. | 
I. ON THE NATURE OF EDESTUS AND RELATED FORMS. 
Notwithstanding the extensive literature concerning the peculiar 
ichthyic remains known as Edestus, Helicoprion and the like, their 
nature, functions, and relations are admitted by most authors to be still 
highly problematical. Occurring as they do singly, and always in the 
detached condition, these objects have been most frequently looked upon 
as Selachian fin-spines, although their correspondence to dental structures 
VOL. XXXIX.— NO. 3 
