78 PALAOZOIC SHARKS 
teeth, shagreen denticles, have proven the antiquity of the 
shark stem and the wealth and variety of its fossil forms ; 
they have provided the evidence that even in Silurian 
times there lived sharks whose exoskeletal specializa- 
tions had progressed further than in their recent kindred : 
that in the Carbon there occurred the culminating-point 
in their differentiation, when specialized sharks existed 
whose varied structures are paralleled only by those of 
existing bony fishes, — sharks fitted to the most special 
environment; some minute and delicate ; others enormous, 
heavy, and sluggish, with er hi and fin spines, and 
elaborate types of dentition. 
But the detached fragments of the fossil sharks can give 
little satisfactory knowledge of their general structures. 
The simpler the form of the shark, indeed, the less liable 
is it to become fossilized. The more generalized of the 
ancient sharks must thus remain structurally unknown 
until more perfect fossils come to be found. To this event 
the discoveries of the past few years have certainly yielded 
most encouraging aid. Several forms of sharks of the 
Lower Carbon and Permian have been obtained in a con- 
dition of admirable preservation, and have already con- 
tributed materially to the morphology of Elasmobranchs. 
Other early forms may be forthcoming which will be found 
to have retained sufficient of the characters of their an- 
cestors to warrant more definite views as to the general 
relationships of fishes. 
Of the three primitive forms of fossil sharks lately 
described: the earliest, from the Ohio Waverly (Lower 
Carbon) is Cladoselache; Dean; a later and puzzling form, 
from the Carboniferous, is Chondrenchelys, Traquair; the 
latest from the Permian and Coal Measures, is P/euracan- 
thus, Agassiz. The only early shark type that had previ- 
