from the Shales of the Northumberland Coal-field. 2375 
tion; and. the lower margin is often produced. into two or more 
fang-like processes, 
In the base of each denticle there is a small fader 
that extends only a short way upwards, and is in direct com- 
munication with the wide medullary canals of the basal por- 
tion, which are for the most part elongated ; but in this respect 
there is considerable variation. The canals are most elongated, 
as might be expected, in elongated specimens. The dentinal 
tubules, which are nearly vertical, are coarse, fasciculated, and 
much branched ; and the osteo-dentine of the base exhibits 
also a few branched tubules, strongest and most numerous 
above and at the margins; below they are comparatively, 
small and obscure. 
A few specimens have occurred which are much elongated 
transversely, and have upwards of twenty denticles ; these are 
oop C. denticulatus of Agassiz. Ctenoptychius is pro- 
ably a dermal tubercle, though it certainly has more the ap- 
pearance of a tooth than either Diplodus or the spined dermal 
tubercles which have been assigned to Gyracanthus. 
. Note-—That Ageleodus diadema of Prof. Owen (pl. 4) is 
the fossil above described cannot for a moment be doubted. 
In general form, size, number and character of the denticles, 
as seen in section, all exactly agree; and there is no difference 
whatever in the histological features: only the specimen 
figured and described in the paper referred to is shorter than 
usual; hence the medullary canals are not so decidedly elon- 
ated as they frequently are. Now no paleontologist would 
Fectate to pronounce our specimens to be Ctenoptychius pecti- 
natus of Agassiz. It is therefore futile to assert that the figure 
of the structure of this genus in the ‘ Poissons Fossiles ’* shows 
“at a glance”’ that it is generically distinct from Ageleodus ; 
and it is certainly erroneous; the difference is merely a dif- 
ference in degree. The medullary canals are more elongated 
and somewhat more regularly parallel in Agassiz’s figure than 
they are in our specimens, in many of which, however, the 
allel and elongated character predominates. In fact there 
is quite as great a difference in this respect between individuals . 
of our suite of specimens as there is between some of them and 
Agassiz’s figure referred to. And it must not be forgotten 
that this figure represents the structure in a different species. 
We repeat, then, that no generic difference is eesopile at a 
glance. M. Agassiz certainly states that the substance at the 
base of the tooth is perfectly homogeneous. In some of our 
specimens, too, the basal portion has lost nearly all traces of 
* Tome iii. pl. M. figs. 4, 5. 
