Carboniferous Limestone Fishes. 519 
difficulty attending the determination of the species of the several 
fossils, and still more of the almost utter impossibility of recon- 
structing, on a sufficiently certain and scientific basis, anything 
approaching a correct idea of the form and parts of the extinct fish. 
In existing sharks the whole framework of the body is frequently 
cartilaginous. The skull and mandible are well developed, but are 
entirely composed of cartilage; the pectoral and pelvic arches are 
the same; the vertebre are in many fishes cartilaginous, in others a 
slight ring of bone is imbedded in the vertebra, 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 frame- 
work 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 eventually imbedded. 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 differences in form as 
to lead to their being placed under separate genera. An interesting 
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 than thirty years considered as separate genera, were ultimately 
found, by the fortunate discovery of a specimen with the teeth un- 
disturbed, to be one and the same genus.! The organs on which 
modern classification is based: the non-decussating optic nerves; 
the muscular conus arteriosus with its varied rows of valvular open- 
ings; and the spiral valve of the intestine, have no existence in a 
fossil state, and it is only by analogy that it can be reasoned that as 
in recent fishes it is found that certain functional relations exist 
between the soft and 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” (pp. 829—880.) 
Of the 59 genera described and figured by Mr. Davis in his 
1 Geol. Surv. Lllinois, vol. i. pp. 88-89. 
