from the Shales of the Northumberland Coalfield. 3871 
a substance resembling shagreen, large patches of which fre- 
tly oceur studded all over with it. One such patch has 
been obtained which measured fifteen inches long and about 
seven inches wide. On this the Diplodi are comparatively 
few in number, and are scattered about. But m another 
patch, of which there are fifty-six square inches, they are very 
numerous, and are crowded together without order. 
There can be little doubt that these shagreen-like patches 
are the remains of the skin of some large fish, and that the 
Diplodi ave dermal tubercles in connexion with it, analogous 
to the spinous tubercles of the Rays. At the same time it 
must be admitted that it is possible enough that the larger 
specimens may have clothed the lips or jaws with a spinous 
... resembling in arrangement the oral armature of the 
ays or Cestracionts; or they may have ranged along the 
back or sides of the body in serial order, as the dermal spines 
frequently do in the Rays; or perhaps they may have been 
scattered here and there among the smaller ones, as is not un- 
frequently the case with such tubercles. : 
 Diplodus has usually three recurved spines, two being large, 
the third quite small; they stand up from a common, rather 
deep, rounded or oval base. The two large or lateral spines 
are ranged side by side; they are stout, conical, and diver- 
gent, both being curved from before backwards, and a little 
compressed in the same direction. ‘The small spine is similar 
in form, and is placed immediately behind the large ones, at 
their basal junction; and in front of them, in a similar posi- 
tion, there is a large, rounded, depressed tubercle. All the 
spines are strongly carinated at the sides from the apex to the 
base; and in well-developed specimens there are two other 
ridges, one in front, the other behind, extending downwards 
for some distance from the apex. : 
These are the normal characters of Diplodus; but it is very 
variable in form. The spines are not unfrequently found stiff 
and short, and much bent and divergent; on the other hand, 
they often occur much elongated, almost parallel, and compa- 
ratively slender. The number of spines also varies; some- 
times there are only two, sometimes only one. When the 
latter is the case, the specimen is usually exposed in profile, 
and the long heel-like projection is well displayed; when, 
however, a complete tubercle is buried in the matrix with 
only one of the lateral spines and its base exposed, the ap- 
pearance is much the same. A tubercle so seen is repre- 
- by M. Agassiz in ‘ Poissons Fossiles,’ vol. iii. tab. 22 6. 
g. 5. 
If Diplodus differs much in form, it also varies greatly in 
