A YORKSHIRE NATURALIST 123: 



bone development, he was not prepared to recognise 

 that each of the minute lacunae of bone was the 

 product of a single cell, though there were other 

 microscopists who accepted that explanation. It 

 was this, along with various similar uncertainties, that 

 led me to pursue my own investigations into the 

 development of bones and teeth. I commenced by 

 making microscopic sections of some of the mam- 

 malian quadrupeds, but discovered nothing beyond 

 what had already been done by Dr. Sharpey. I then 

 turned to bones, teeth, and scales of fishes, both 

 recent and fossil, from which I obtained much more 

 valuable and interesting results. I began by pre- 

 paring microscopic sections of a large number of 

 fossil scales, in some of which I found structures of 

 remarkable beauty and interest. 



M. Agassiz on commencing his celebrated re- 

 searches amongst the fossil fishes of the world 

 found that the existing classifications of living forms 

 were inapplicable to the fossil ones. Hence he pro- 

 ceeded to construct a new one better adapted to his 

 object. He established four groups based upon 

 peculiarities in the structure of their scales. These 

 were the Ganoids, Placoids, Cycloids, and Ctenoids. 



The Ganoids were the most highly organised of 

 these classes. In them the scales were rhomboidal 

 in form, thick and hard in substance, and, moreover, 

 had their outer surface covered with a layer of bright, 

 shining enamel. Few fishes now living belong to 

 this group, but it is well represented by the huge 



