ON SOME ARMOURED FISHES. 
17 
seems probable that the rest of the body was only covered with 
a tough skin. Like the modern sheat-fish of the Granges, 
it may have protected itself by burrowing in the mud and 
watching for its prey, with only its mail-clad parts exposed. 
The powerful dentition of Dinichthys is suggestive of its 
carnivorous habits ; and being so heavily weighted by the thick 
shields encasing its vital organs, it would be compelled to obtain 
food rather by cunning than by swift pursuit. These heavily 
armed Placoderms had only a comparatively short range in 
time, remains of the group being found only in the Upper 
Silurian and Devonian rocks. Thus it seems, as though unable 
to cope in the struggle for existence with the lighter armed and 
more active race of ganoids which predominated in the Devonian 
waters, they died out, leaving no immediate descendants.* 
Dinichthys , like Goccosteus , appears to have had an unossi- 
fied notochord. 
But besides the “ Lancelet ” (. Amphioxus ) — the most lowly 
organized of existing vertebrates — three genera, belonging to 
the highest order of fishes — the Dipnoi (or double-breathers) — 
living at the present day, all exhibit the same embryonal struc- 
ture in the adult state, namely, the Lepidosiren found in the 
Amazons, South America, the Protopterus of the African 
rivers, and the Ceratodus (Plate II., Fig. 2) of the Australian 
rivers. 
These remarkable fishes have the swim-bladder modified 
into true internal lungs, whilst they retain internal branchiae, so 
that they are capable of existing either in the water or on land. 
During the rainy season, large rivers like the Amazons and the 
Gambia are subject to overflow, whilst those of the Australian 
continent are not only liable to overflow, but are subject to 
long continued periods of drought, during which the waters 
sink so low that the beds of the rivers are frequently dry, 
and water is only found in deep hollows ( c waterholes ’) or in 
muddy pools. The Protopterus and Lepidosiren bury them- 
selves in the mud, and breathe by means of their lungs 
until the return of the rainy season sets them free once 
more to pursue an aquatic existence. The Ceratodus , if 
unable entirely to subsist without water, can nevertheless 
live in water so muddy as to be unfit for branchial re- 
spiration, and to come to the surface and breathe air direct 
bv means of its modified swim-bladder. But Dr. GHintherf 
* A. Crane, “Geol. Mag.” 187 7, Yol. iv. p. 214. 
t “ Description of Ceratodus , a Genus of Ganoid Pishes, recently dis- 
covered in the Rivers of Queensland, Australia.” By Dr. A. Gunther 
P.R.S. “ Phil. Trans.” 1871 . Part II. p. 511. 
NEW SERIES, VOL. II. — NO. V. C 
