THE OLD RED SANDSTONE. 85 
Diplacanthus ; and, though the smallest ichthyolite of the for- 
mation yet known, it is by no means the least curious. The 
‘ength from head to tail, in some of my specimens, does not 
exceed three inches; the largest fall a little short of five. 
The scales, which are of such extreme minuteness that their 
peculiarities can be detected by only a powerful glass, re- 
semble those of the Cheiracanthus ; but the ridges are more 
waved, and seem, instead of running in nearly parallel lines, 
to converge towards the apex. There are two dorsals, the 
one rising immediately from the shoulder, a little below the 
nape, the other directly opposite the anal fin. The ventrals 
are placed near the middle of the belly. There is a curious 
mechanism of shoulder-bone involved with a lateral spine and 
with the pectorals. The creature, unlike the Cheiracanthus, 
seems to have been furnished with jaws of bone: there are 
fragments of bone upon the head, tubercled apparently on 
the outer surface ; and minute cylinders of carbonate of lime 
running along all the larger bones, where we find them acci- 
dentally laid open, show that they were formed on internal 
bases of cartilage. But the best marked characteristic of 
the creature is furnished by the spines of its fins, which are 
of singular beauty. Each spine resembles a bundle of rods, 
or, rather, like a Gothic column, the sculptured semblance of 
a bundle of rods, which finely diminish towards a point, sharp 
and tapering as that of a rush. (See Plate VIII., fix 4.) * 
The rest of the fin presents the appearance of a mere scaly 
membrane, and no part of the internal skeleton appears. 
Perhaps this last circumstance, common to all the ichthyolites 
of the formation, if we except the families of the Coccosteus 
—- 
* Agassiz reckons four species of Diplacanthus — D. crassispinus, 
D. longispinus, D. striatulus, and D. striatus. 
GQ * 
