CLASSIFICATION OF DEVONIAN FISHES 457 
of Pander, such as Asterolepis (Ag. and Miller) really belong to the 
same group as Coccosteus and Pterichthys, or not, is a question which 
can perhaps be hardly settled at present ; although, provisionally, I am 
much inclined to associate them together. In principle, the cranial 
structure of Asterolepis is very similar to that of Coccosteus. 
Having disposed of the undoubted Elasmobranchs, of the Crosso- 
pterygian Ganoids, and of the “ Placodermi” of the Devonian epoch, 
several important and rather difficult groups remain for discussion. 
These are the Acanthodide, the genera Cephalaspis and Preraspis, and 
the genus Chezrolepis. 
The ACANTHODID< have hitherto been ranked among the Ganoids, 
but the following considerations have often led me strongly to suspect 
that they might be Elasmobranchs : — 
1. Their dorsal spines are similar in form and mode of implanta- 
tion to those of the Elasmobranchii, except perhaps that the surface 
of the implanted portion is less different from the rest than in the 
latter order. 
2, Their dermal ossicles are more like shagreen than scales. 
3. As Roemer has pointed out, their lateral line runs between two 
rows of these ossicles, and is not formed by separate canals or grooves 
in successive scales as in most Ganoids and Teleosteans. 
4. They seem to have had no distinctly ossified cranial bones. 
5. They have no opercular apparatus, but as Sir Philip Egerton 
long ago pointed out to me, their branchial arches are naked. 
6. The sternal part of their pectoral arch seems to have had no 
bony connexion with the head. 
On the other hand, however, it must be considered that,— 
1. The Acanthodide, unlike all Elasmobranchs, have great spines 
articulated with the pectoral arch. 
2. The dermal plates of the Ganoid Chezrolepis are very shagreen- 
like, though affirmed by Pander to differ in structure from those of 
Acanthodide. 
3. The cranial bones become less and less developed in the Chon- 
drosteous Ganoids, until in Spatularta they are very thin squamose 
lamella ; so that there is no great difficulty in the way of supposing 
their entire absence in a true Ganoid. 
4. In the same way, the opercular apparatus, small in Acczpenser, 
is still more reduced in Spatu/artza. 
5. The thin, curved, toothless mandibles of Sfatwlaria present, 
perhaps, the nearest analogue to the singular mandibular bones of 
Acanthodes, 
