THE LOWER COAL-MBASURES, 187 
to be terminal. The walls of this cavity, forming the base of the 
spine, were thin, and in the specimen they have been crushed by the 
superincumbent matrix. The upper half of the spine retains its 
normal form, is circular, curves more rapidly, and gradually di- 
minishes in diameter toa blunt point. From the point, extending 
about two inches along the dorsal aspect, there is a double row of 
eight or nine widely separated protuberances or denticles. In the 
lower part the denticles are ‘2 inch in breadth, but nearer the 
apex they gradually diminish to about half that size ; they are broad 
at the base, laterally compressed, and rapidiy converge to a very 
obtuse point. They are separated by spaces cecupying a rather 
larger area than the denticles. The latter occur alternately, the 
projection on one side of the spine being oppesite to the depression 
on the other. 
The spine was enumerated in the list of fish-remains from the 
Bone-beds (No. 11). At that time, I referred to it as possessing only 
one row of denticles on the posterior face. Since that was written 
I have ascertained that there is a double row. 
From the genus Orthacanthus, which appears to approach it 
nearest in general character, this Ichthyodorulite presents con- 
siderable differences in its general outline, and more especially in the 
peculiar form and large size of the posterior denticles. Orthacanthus, 
as defined by Prof. Agassiz, is a straight spine ; this one is slightly 
curved. Mr. J.S. Newberry, in the Report on the paleontology of 
Ohio, has described and figured a curved spine, Orthacanthus arcuatus, 
which he considers identical in structure with Prof. Agassiz’s O. 
cylindricus, and consequently has united withit in the same genus, 
recent discoveries merely proving that the name was not well 
chosen. 
Orthacanthus arcuatus is, however, very distinct from the spine 
I am describing. The greatest discrepancy is in the form of 
the denticles ranged on either side of the dorsal aspect of the 
spine. In Orthacanthus cylindricus, Agass., and in each species 
since described, the teeth are more or less hooked, pointed and close 
together ; but in this instance none of these characteristics will apply. 
The denticles are widely separated, rounded, and blunt; to such an 
extent is this the case, that the term denticle is scarcely applicable, 
as they have more the character of waving projections alternately pro- 
duced, first right then left, from the dorsal surface of the spine. In 
consideration of this peculiarity, I suggest that the spine receive the 
generic and specific name of Phricacanthus biservalis. 
EXPLANATION OF PLATE X. 
Figs. 1-8. Teeth of Plewrodus affinis, Ag. 
Figs. 9-11. Spines of Plewrodus affinis, Ag. 
Figs. 12-14. Spines of Hoplonchus elegans. 
Fig. 15. Spine of Ctenacanthus equistriatus. 
Fig. 16. Spine of Phricacanthus biserialis. 
Fig. 17. Anterior extremity of fig. 16 separated from the matrix, 
