330 On the Fossil Fishes cf the Carboniferous Limestone Series of Great Britain. 
vertebree and there is further modification in the direction of a completely osseous 
centrum. The teeth, the spines placed in front of the dorsal or pectoral fins or 
other parts of the body, and the dermal tubercles or shagreen, are the only parts 
of the fishes which are composed of an osseous or other hard substance which would 
be capable of resisting speedy decomposition after death. As might be naturally 
inferred, these are the only parts of the fishes of Carboniferous age which are found 
fossil ; even the vertebree appear to have been entirely devoid of calcareous deposit 
and with the remaining cartilaginous portions of the fish have been decomposed and 
lost. The cartilaginous framework which held the teeth and spines together having 
decayed, these less destructible organs speedily became separated, and may have 
been carried considerable distances apart by currents or tides before they were eventu- 
ally embedded. it is an extremely rare occurrence to find the various teeth, spines, 
and dermal tubercles, in such relationship that they can be identified as belonging 
to the same fish. Not only is this the case with the separated spines and teeth, 
but the greatest confusion may, and no doubt does to a large extent, exist in the 
determination of the many forms of teeth, and it may easily happen that the teeth 
which have lain side by side in the palate of one fish, may be considered by the 
ichthyologist as representing not only different species but present so marked differ- 
ences in form as to lead to their being placed under separate genera. An interest- 
ing example of this kind, in which the two genera Cochliodus and Helodus, instituted 
by Professor Agassiz, though they had been found in several countries in Europe as 
well as in America, and for more thanthirty years considered as separate genera, were 
ultimately found, by the fortunate discovery of a specimen with the teeth undis- 
turbed, to be one and the same genus.* The organs on which modern classifica- 
tion is based :—the non-decussating optie nerves; the muscular conus arteriosus 
with its varied rows of valvular openings; and the spiral valve of the intestine, 
have no existence in a fossil state, and it is only by analogy thatit can be reasoned 
that as in recent fishes it is found that certain functional relations exist between 
the soft and the hard parts of the fishes, so having procured the hard bony disjecta 
membra of the extinct fishes, and these exhibiting certain relationships with the 
recent forms, it may be inferred that the more perishable portions have also borne 
a similar relationship to those of recent forms. 
The fishes of the Mountain Limestone, excepting Ganoids, are comprised in the 
sub-order Plagiostomata of Giinther, which includes the sharks and rays, but 
excludes the chimeroid fishes, and for this reason the sub-order as defined by Dr. 
Giinther is perhaps preferable to the Elasmobranchii of Professor Huxley in con- 
sidering the fishes of Carboniferous age ; the Chimeree being of Jurassic, Cretaceous, 
and Tertiary age, unless the bones described by Newberry in “The Geology of Ohio,” 
Vol. J., page 807, as Rhynchodus should be proved to carry the origin of the 
group so far back as Carboniferous times. 
*« Geological Survey of Illinois,” Vol. I1., pp. 88-89. 
