560 
DR. A. GUNTHER’S DESCRIPTION OE CERATODUS. 
other “ Crossopterygians.” Fin-rays in definite numbers, joined to the interspinous 
bones, obtuse paired fins as in Polypterus, a double pelvis, developed upper jaws with 
small conical teeth, external nostrils, are characters sufficient to prevent us from asso- 
ciating Ceratodus and its allies with the Ccelacanths, which form not only a distinct 
family, but belong, according to my view, to a distinct suborder*. 
Concluding Remarks. 
The great interest attached to the discovery of Lepidosiren lay in the combination of 
elements of its organization denoting an approach to the Amphibian type, with others 
which remind us of embryonic stages of development. The supposition of some zoologists, 
who saw in Lepidosiren an instance of the latest step of advancement attained by the 
struggling ichthyic type towards the higher class, that of Amphibians, is not con- 
firmed ; for we find that the Dipnoi reach back, with comparatively insignificant modifi- 
cations, into one of the oldest epochs from which fish-remains are preserved. The modi- 
fication which would appear to demand our special attention is the condition of the 
tail, heterocercy being generally taken to be a lower degree of development than diphy- 
cercy. Thus one might suppose that the heterocercal Dipterus of the Devonian epoch 
is the less highly organized ancestor of the diphycercal Lepidosiren and Ceratodus; but 
then, on the other hand, Plainer opleur on, and also Tristichopterus , prove that diphycercy 
is a condition attained even at that early time by fishes more or less closely allied to the 
Dipnoi. 
The defenders of the doctrine of evolution hold that a space of time like that which 
elapsed from the Devonian epoch to our period, is as a drop in the sea, when compared 
with the time required for the development of organic life. From this point of view the 
Ganoid fauna of the Devonian and even the Pteraspis of the Upper Ludlow formation, 
must have been preceded by long and varied ichthyic series. Tristichopterus with its 
osseous vertebral column may have been the surviving representative of a Ganoid sub- 
order then extinct, as Polypterus has been regarded as the sole survivor of Crossoptery- 
gians ; and the Dipnoous type, as it appears to us for the first time in the Devonian 
epoch, was not the beginning of a series, but the last of many preceding developmental 
stages. Future discoveries in hitherto unexplored formations may prove these suppo- 
sitions to be quite correct ; but when we limit ourselves for the present to the actual 
evidence before us, we find it consists of the following facts only : — The Dipnoous type 
is represented in the Devonian and Carboniferous epochs by several genera ( Dipterus , 
Cheirodus, Conchodus, Plainer opleur on) ; it is then lost down to the Trias and Lias, where 
the scanty remains of a distinct genus, Ceratodus , testify to its presence ; no further 
trace of it has been found until the present period, where it reappears in three genera, 
one of which is identical with that of the Mesozoic era. Now at present scarcely any 
* I do not attach any value to the terms subfamilies, families, suborders, &c., except as expressions of the 
relative degree of affinity ; and in the preceding notes I have used them in accordance with the synoptical table 
published by Professor Huxley. 
