Sturioniens. Plagiostomes. Cyclostomes.
We have only to compare this system with that of Linnaeus if we wish to
measure the gigantic stride made by ichthyology during the intervening
period of seventy years. The various characters employed for
classification have been examined throughout the whole class, and their
relative importance has been duly weighed and understood. The important
category of "family" appears now in Cuvier's system fully established as
intermediate between genus and order. Important changes in Cuvier's
system have been made and proposed by his successors, but in the main it
is still that of the present day.
Cuvier had extended his researches beyond the living forms, into the
field of palaeontology; he was the first to observe the close
resemblance of the scales of the fossil _Palaeoniscus_ to those of the
living _Polypterus_ and _Lepidosteus_, the prolongation and identity of
structure of the upper caudal lobe in _Palaeoniscus_ and the sturgeons,
the presence of peculiar "fulcra" on the anterior margin of the dorsal
fin in _Palaeoniscus_ and _Lepidosteus_, and inferred from these facts
that the fossil genus was allied either to the sturgeons or to
_Lepidosteus_. But it did not occur to him that there was a close
relationship between those recent fishes. _Lepidosteus_ and, with it,
the fossil genus remained in his system a member of the order of
_Malacopterygii abdominales_.
Agassiz.
It was left to L. Agassiz (1807-1873) to point out the importance of the
structure of the scales as a characteristic, and to open a path towards
the knowledge of a whole new subclass of fishes, the _Ganoidei_.
Impressed with the fact that the peculiar scales of _Polypterus_ and
_Lepidosteus_ are common to all fossil osseous fishes down to the Chalk,
he takes the structure of the scales generally as the base for an
ichthyological system, and distinguishes four orders:--
1. _Placoids._--Without scales proper, but with scales of enamel,
sometimes large, sometimes small, and reduced to mere points (Rays,
Sharks and Cyclostomi, with the fossil Hybodontes). 2.
_Ganoids._--With angular bony scales, covered with a thick stratum of
enamel: to this order belong the fossil Lepidoides, Sauroides,
Pycnodontes and Coelacanthi; the recent Polypterus, Lepidosteus,
Sclerodermi, Gymnodontes, Lophobranches and Siluroides; also the
Sturgeons. 3. _Ctenoids._--With rough scales, which have their free
margins denticulated: Chaetodontidae, Pleuronectidae, Percidae,
Polyacanthi, Sciaenidae, Sparidae, Scorpaenidae, Aulostomi. 4.
_Cycloids._--With smooth scales, the hind margin of which lacks
denticulation: Labridae, Mugilidae, Scombridae, Gadoidei, Gobiidae,
Muraenidae, Lucioidei, Salmonidae, Clupeidae, Cyprinidae.
If Agassiz had had an opportunity of acquiring a more extensive and
intimate knowledge of existing fishes before his energies were absorbed
in the study of fossil remains, he would doubtless have recognized the
artificial character of his classification. The distinctions between
cycloid and ctenoid scales, between placoid and ganoid fishes, are
vague, and can hardly be maintained. So far as the living and
post-Cretacean forms are concerned, he abandoned the vantage-ground
gained by Cuvier; and therefore his system could never supersede that of
his predecessor, and finally shared the fate of every classification
based on the modifications of one organ only. But Agassiz opened an
immense new field of research by his study of the infinite variety of
fossil forms. In his principal work, _Recherches sur les poissons
fossiles_, Neuchâtel, 1833-1843, 4to, atlas in fol., he placed them
before the world arranged in a methodical manner, with excellent
descriptions and illustrations. His power of discernment and penetration
in determining even the most fragmentary remains is astonishing; and, if
his order of Ganoids is an assemblage of forms very different from what
is now understood by that term, he was the first who recognized that
such an order of fishes exists.
The discoverer of the _Ganoidei_ was succeeded by their explorer
Johannes Müller (1801-1858). In his classical memoir _Über den Bau und
die Grenzen der Ganoiden_ (Berl., 1846) he showed that the Ganoids
differ from all the other osseous fishes, and agree with the
Plagiostomes, in the structure of the heart. By this primary character,
all heterogeneous elements, as Siluroids, _Osteoglossidae_, &c., were
eliminated from the order as understood by Agassiz. On the other hand,
he did not recognize the affinity of _Lepidosiren_ to the Ganoids, but
established for it a distinct subclass, _Dipnoi_, which he placed at the
opposite end of the system. By his researches into the anatomy of the
lampreys and _Amphioxus_, their typical distinctness from other
cartilaginous fishes was proved; they became the types of two other
subclasses, _Cyclostomi_ and _Leptocardii_.
Müller proposed several other modifications of the Cuvierian system;
and, although all cannot be maintained as the most natural arrangements,
yet his researches have given us a much more complete knowledge of the
organization of the Teleostean fishes, and later inquiries have shown
that, on the whole, the combinations proposed by him require only some
further modification and another definition to render them perfectly
natural.
The discovery (in the year 1871) of a living representative of a genus
hitherto believed to be long extinct, _Ceratodus_, threw a new light on
the affinities of fishes. The writer of the present article, who had the
good fortune to examine this fish, was enabled to show that, on the one
hand, it was a form most closely allied to _Lepidosiren_, and, on the
other, that it could not be separated from the Ganoid fishes, and
therefore that _Lepidosiren_ also was a Ganoid,--a relation already
indicated by Huxley in a previous paper on "Devonian Fishes."
Having followed the development of the ichthyological system down to
this period, we now enumerate the most important contributions to
ichthyology which appeared contemporaneously with or subsequently to the
publication of the great work of Cuvier and Valenciennes. For the sake
of convenience we may arrange these works under two heads.