CLASSIFICATION OF DEVONIAN FISHES 425 
Practical Geology shows that some of the teeth, at any rate, were 
of large size, and longitudinally grooved at their bases. 
Only three specimens of Glypfopomus are at present known, and 
no one of these exhibits either the paired or the median fins; but 
the close correspondence of the cranial structure of this genus with 
that exhibited by Glyptolemus, leaves no doubt on my mind that, 
when discovered, the fins will be found to be similar, in all essential 
respects, to those of the latter genus (see note, p. 460). The sharply 
rhomboidal scales are thicker in proportion than those of any other 
Devonian fish, and are pitted upon their surfaces like the scutes of 
the Crocodilia. 
As has been seen, the angles of the scales of Gyroptychius are 
apt to become rounded off, so as to present a transition from the 
rhomboid to the cycloid contour, and, hence, it is less surprising than 
it seems at first sight, to find fishes with eminently cycloid scales, 
so similar, in all the essential features of their organization, to 
Glyptolemus, Gyroptychius, and Glyptopomus, as imperatively to 
demand a place near them in any natural arrangement. 
Holoptychius (Agassiz), for example, has a depressed head 
(though deeper than that of Glyptolemus), and a  conically 
tapering caudal extremity; the orbits are situated far forwards 
and the gape extends far back. The frontal bones are distinct 
from one another, and from the parietals, which last are large 
and co-adapted, though quite distinct; the occiput is covered in 
by three bones, a median and two lateral; there are two prin- 
cipal and a number of lateral jugular plates, and there is no rhom- 
boidal median jugular plate interposed between the principal 
jugulars. Some of the teeth are larger than the others, and longi- 
tudinally striated at their bases. The paired fins are very acutely 
lobate, and there are two dorsal fins placed in the posterior half of 
the body. The ventral fins are situated under the first dorsal, and 
are succeeded by a single anal. 
Thus far, the reader who compares this description with that of 
Glyptolemus already given, will find the two essentially identical. 
But the tail of Holoptychius differs from that of Glyptolemus, in 
that it is little more than semi-rhomboidal, the upper moiety 
being far less developed than the lower,! and the scales are, 
1 In my restoration of Holoptychius (Dr. Anderson’s ‘‘ Dura Den,” p. 69) I have repre- 
sented the fish with a diphycercal tail ; but I am now prepared to admit that the evidence 
on which I rested this conclusion was not trustworthy, and that Sir Philip Egerton’s view of 
the case is in all probability correct. However, I must say, that I have never yet seen a 
Holoptychius with its caudal extremity in a perfectly satisfactory state of preservation. 
