from the Shales of the Northumberland Ooal-field. 373 
is in part a natural cavity, frequently seen in sections. A. wn- 
datus (pl. 10) is a lateral section of a single minute spine of 
the same variety of Diplodus, somewhat abnormal in form. 
There is no difference of importance in the minute structure, 
and it exhibits in a most distinct manner the numerous con- 
centric layers of dentine mentioned by M. Agassiz as charac 
teristic of Diplodus (vol. iii. p. 209). 
Diplodus has supplied Prof. Owen with still another generic 
form, which is the fourth based upon this variable fossil. 
Ochlodus (pl. 5) is nothing more than one of the large varieties 
of this dermal tubercle, crushed laterally—a variety, probably, 
having originally one of the large spines smaller than the 
other. A figure of such a tubercle is given by Mr. Binney 
in the paper before quoted*. From the representation of 
Ochlodus it is evident that the specimen has been crushed: the 
dentinal walls are cracked in several places, the upper wall has 
been forced in upon the osteo-dentine of the pulp-cavity, and 
the continuity of the tissue of the spines has been severed ; 
the osteo-dentine of the pulp-cavity has, in a great measure, 
been displaced, and the base shattered to fragments. All these 
appearances are shown in a section now before us, which was 
made of a specimen crushed laterally or a little diagonally, 
and which closely resembles in size and contour Ochlodus. 
It is evident, too, that much of the fractured base in this genus, 
and also a considerable portion of the two smaller spines, have 
been removed in making the section. | 
_ The thickness of the dentine and the size of the pulp-cavity 
are very variable features in Diplodus. Hven in the same spe- 
cimen the peripheral dentine occasionally varies considerably at 
different parts of the circumference, as may be seen on making 
a transverse section of the spines; and as they are com- 
pressed, as we have stated above, the relative size of the pulp- 
eavity varies with the plane of the section. This is one source 
of variation; but were the pulp-cavity quite cylindrical, or 
rather circular in transverse section, its apparent relative pro- 
portion to the dentinal wall would depend upon the degree of 
eccentricity of the section. The pulp-cavity is consequently 
found to vary extremely in size in Diplodus. In the crushed 
specimen we have spoken of, this cavity is quite as large as it 
is represented in the figure of Ochlodus; and, again, in other 
specimens it is no larger than we see it in the figure of the so- 
called Pternodus productus. 
The acute points represented in the section of Ochlodus are 
not the apices of the spines as believed by Prof. Owen; the 
true apices have all been removed in making the section 
* Trans. of the Manchester Geol. Soc. vol. i. pl. 5. fig. 17. 
