78 PALAEOZOIC 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 stout head 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 Cladoselachc, Dean ; a later and puzzling form, 

 from the Carboniferous, is CJioiidreiichelys^ Traquair ; the 

 latest from the Permian and Coal Measures, is Pleicraciui- 

 thiis^ Agassiz. The only early shark type that had previ- 



