
NX 
Sept. 28, 1871] 
NATURE 

On the other hand they agree with the Ganoids in 
_ having, in addition to the ordinary two divisions of the 
fish-heart, a third contractile chamber. This bulbus 
arteriosus is very different from the Bulbus aorte of the 
Teleosteous fishes, where it is simply a swelling of the 
walls of the aorta, not contractile, without valves in the 
interior, and separated from the heart by two valves oppo- 
site to each other. If this remarkable arrangement is 
deemed (and, I think, very justly) to be sufficient to sepa- 
rate the living Ganoids as a sub-class from the Te/eoste¢, 
it is certainly significant enough to suggest the union 
of the Ganoids with the Plagiostomes. Moreover, this 
character is supported by two others of great importance, 
viz., the presence of a spiral valve in the intestine, which 
is found in a more or less developed state in all Ganoids, 
Sharks, and Rays, but is entirely absent, even in a rudimen- 
tary condition, in the Teleostei, and by the optic nerves 
being placed side by side, and not decussating,as is the case 
in all our ordinary fishes. Of the characters connecting 
these fishes I will refer to one other, as it has been de- 
scribed above, namely, that the fore and hind limbs of 
the Plagiostomes are also paddles supported by a car- 
tilaginous structure, as in the Dzfxoz. 
The evidence in favour of a union of Ganoids with 
Plagiostomesis rendered complete bythe Chimeras, which 
hold a surprisingly intermediate position. They are Sharks 
in external appearance and with regard to the structure 
of their organs of propagation; they are provided with 
the same copulatory organs, and their ova are large, en- 
closed in a horny case, and provided with adhesive appen- 
dages. Many species of Sharks, when in a very young 
state, are provided with a double dorsal series of spines 
(permanent in certain Rays), which are lost with age ; and 
this most remarkable developmental character occurs 
likewise in young Chimeeras. On the other hand, there is 
only one external gill-opening on each side, as, for 
instance, in Cevatodus, which, on the other hand, shows 
the first step towards a coalescence of the gills with the 
walls of the gill-cavity. The skeleton is notochordal, and 
the palatal and maxillary apparatus coalesce with the skull, 
as in Dipuoz, which is not the case in any Plagiostome ; 
likewise the dentition approaches that of Ceratodus. 
Finally, Sir P. Egerton has drawn attention to the most 
important fact that the dorsal spine is articulated to a 
neural apophysis, and not merely implanted in the soft 
parts and immovable, as in Sharks, 
Thus, then, the union of these fishes in one sub-class 
appears to me fully justified, as far as the living forms 
are concerned ; but, as is implied by the name Pa/e- 
ichthyes, which I have proposed for this sub-class, it is 
intended to comprise also a great variety of forms from 
the Paleozoic Era, in fact, the predecessors of the 
Teleostean fish-fauna of the present period. I am aware 
of the objections that may be urged. First, it may appear 
to some to be an improper proceeding to unite in the same 
sub-class fishes of so different an appearance as a Shark 
and Lepidosiren, or as an Amia and a Pteraspis ; but let 
them consider what a comprehensive category a sub-class 
necessarily is—that the diversity between the fishes just 
named is not greater than that existing between a Sun-fish 
(Orthagoriscus) and an eel, or between a viviparous Em- 
biotoca and a Loricaria, forms admitted by every ichthyo- 
logist of the present day as members of the same sub- 
class, that of Teleosteans. In fact the Palzichthyes are 
composed of a similar series of modifications as the 
Teleosteans, some of the members of one sub-class exhibit- 
ing marked analogies with those of the other, inthe same 
manner as is the case with Placentalia and Implacentalia 
among mammals. To mix up ganoid-looking Teleosteans, 
like the Siluroids, with Ganoids, is as little in accordance 
with the advanced state of our ichthyological knowledge, as 
the union of Salamandra with Lacerta would be. Secondly, 
other naturalists may consider it very hazardous to estab- 
lish a division, of which the majority of members are extinct 

and known from remains of the hard parts only, and to 
characterise it by peculiarities of the soft parts. But 
why should we not make use of zoological evidence 
for the completion of the imperfect paleontological 
record, with the same benefit to science as in other cases, 
since not a few zoological problems have been, or can 
only be, solved by reasoning founded on palzontological 
facts? If, in the determination of affinities, we were to 
limit ourselves only to the consideration of those parts 
which have been preserved in the process of fossilisation, 
we could never expect any other result but the creation 
of most artificial assemblages of forms, although the 
characters of some natural families, or even orders, might 
be partly recognised. 
On the one hand, we know that all the Teleosteous 
fishes, that is, the types which are predominant in the 
present and next preceding epochs, and which were but 
sparingly (Coccosteus ?) represented in the Palzozoic, if 
they existed at all, agree, in spite of all other differences, 
with one another in possessing a two-chambered heart, 
with a rigid bulbus aorta: and decussating optic nerves, 
and in never exhibiting a trace of a spiral valve in the 
intestine.* On the other hand, we find that the few ichthyic 
types which have survived from the Paleozoic Era into 
our period, and those of which no immediate repre- 
sentative is known in that Era, but which approach that 
Amphibian fish-type by unmistakeable characters, agree, in 
spite of all other differences, in having a three-chambered 
heart, non-decussating optic nerves, and a spiral valve in 
the intestine. These are facts ; and it seems to be a fair 
conclusion that the members of the Paleozoic fish-fauna 
had essentially the same organisation of those soft parts 
as their surviving representatives. 
In conclusion I may shortly pass in review the living 
Paleichthyes, especially in regard to their distribution 
over the globe. 
1, Of the order Plagiostomata or Marine Paleichthyes, ~ 
140 species of Sharks, distributed among 39 genera, are 
known, and 150 species of Rays, belonging to 25 genera. 
They inhabit nearly all the seas of the globe, decreasing 
in number from the tropics towards the poles. Only very 
few enter or live in freshwater. 
2. The order Holocephala contains only four species, 
viz., three Chimzeras and one Callocephalus ; they are re- 
stricted to the seas of the temperate zones of both hemi- 
spheres, and are absent between the tropics. 
3. The order Ganotdet or Freshwater Paleichthyes is 
represented by one species of Azza, from North America ; 
three species of Lefzdosteus, from the same region, but 
extending southwards into Central America and Cuba 3 
two species of Polypterus (Calamoichthys) from the tropi- 
cal parts of Africa ; two species of Polyodon, from the 
Mississippi and the Yantsekiang; about twenty-five 
Sturgeons, from the temperate and sub-arctic regions of 
the Northern Hemisphere ; two species of Ceratodus from 
tropical Australia ; one species of Lefzdoszren from the 
Amazon river, and one of Protopterus from tropical Africa. 
Although the majority of the Sturgeons pass a part of the 
year in the sea, they must be regarded as freshwater 
fishes like the migratory Salmones, because they deposit 
their spawn in the rivers, where they also pass the first 
period of their growth ; some species never enter the sea 
at any period of their life, and none are known to propa- 
gate in the sea. 
The total number of fishes known at present being 
about 9,000, the Pa/eichthyes form only 3°6 per cent. of 
thatnumber. But from the extent of the regions hitherto 
ichthyologically unexplored, and from the numerous addi- 
tions annually made to the list of known forms, I do not 
believe that we are acquainted with much more than one- 
tenth of the species of fishes actually existing. 
ALBERT GUNTHER 
* From these considerations A mphioxrus and the Marsipobranchii are ex- 
cluded, the former being evidently the type of a distinct sub-class. 
429 
