332 J. w. DAVIS ON THE genus plefeacanthfs, agass.
come finer or altogether disappear near the point. On the posterior

face there is a double row of obtusely pointed denticles two tenths

Fig. 8. — PleuracantJius cylindricus, Davis.
a. Section of spine, b. Portion of posterior surface, showing denticles.
c. Denticles, enlarged.
of an inch apart, except near the apex (where the space between
them is much reduced), slightly curved towards the base. They ex-
tend from the point downwards 7^ inches, and increase somewhat
irregularly in size with the diameter of the spine or towards the
base. The denticles are firmly implanted, round near their base,
but contract and form a cone elongated transversely to the longitu-
dinal axis of the spine ; so that whilst the points are towards the
base, they are also turned decidedly away from the centre of the
spine (fig. 8 b). In the specimen figured by M. Agassiz*, a median
ridge is represented along the posterior face, between the two rows
of denticles, and continuing the whole length of the spine. In the
specimens I have examined this does not occur : there is sometimes
a level surface between the denticles ; but more frequently there is a
very decidedly hollow groove, which rarelj', however, extends far
beyond the termination of the denticles.
A section of another spine is represented in fig. 8 a, in which,
the denticles are very widely separated. It is of the same species as
the specimen described above ; and aU the intermediate stages may
Tdo traced. In this instance the spine is of the same diameter as the
one represented in fig. 8 6, whilst the denticles are four tenths of an
inch apart, or double the distance. There is also in this example a
most decided groove, equidistant between the rows of denticles, and
quite one tenth of an inch across.
Locality. T^ot uncommon in the Coal-measures.
I am indebted to Mr. John Ward for placing at my disposal the
beautiful specimens from which the above descriptions have been
principally derived. They are from the ironstone shale at Fenton,
in Staffordshire. They are much larger and in better preservation
ihan any I have seen from the Yorkshire Coal-field.

• Op. cit.