32 PROGRESS OF PALEONTOLOGY n 



impressed the popular imagination, the recon- 

 struction of an extinct animal from a tooth or a 

 bone, is based upon the simplest imaginable appli- 

 cation of the logic of Steno. A moment's con- 

 sideration will show, in fact, that Steno's conclu- 

 sion that the glossopetrae are sharks' teeth implies 

 the reconstruction of an animal from its tooth. It 

 is equivalent to the assertion that the animal of 

 which the glossopetrae are relics had the form and 

 organisation of a shark ; that it had a skull, a 

 vertebral column, and limbs similar to those which 

 are characteristic of this group of fishes ; that its 

 heart, gills, and intestines presented the pecu- 

 liarities which those of all sharks exhibit ; nay, 

 even that any hard parts which its integument 

 contained were of a totally different character 

 from the scales of ordinary fishes. These conclu- 

 sions are as certain as any based upon probable 

 reasonings can be. And they are so, simply be- 

 cause a very large experience justifies us in 

 believing that teeth of this particular form and 

 structure are invariably associated with the pecu- 

 liar organisation of sharks, and are never found 

 in connection with other organisms. Why this 

 should be we are not at present in a position even 

 to imagine ; we must take the fact' as an empirical 

 law of animal morphology, the reason of which 

 may possibly be one day found in the history of 

 the evolution of the shark tribe, but for which it 

 is hopeless to seek for an explanation in ordinary 



