A YORKSHIRE NATURALIST 129* 



development of the skeleton of the vertebrate 

 animals from this point of view, progression mani- 

 fests itself conspicuously. In every case the produc- 

 tion of chondriform bone indicates an earlier state of 

 organisation than membraniform bone does. Sharks 

 and rays, the placoid fishes of Agassiz, have long 

 been known as the cartilaginous fishes; but their 

 skeletons are not, as the name might imply, mere 

 unaltered cartilage; this tissue is converted into 

 chondriform bone. But in one of this tribe the 

 sawfish we find the bones of the snout flattened,, 

 immensely elongated, and carrying a row of the huge 

 teeth along each of its margins, to which the animal 

 owes its popular name. This powerful aggressive 

 organ would not be sufficiently rigid for its purpose 

 were it solely composed of chondriform bone. Such 

 bone is present in an inner layer, but a second layer 

 of exostial bone is added externally. This is formed 

 of a soft layer that is somewhat midway between 

 true membrane and cartilage, in which small verti- 

 cally disposed rods of hard tissue are developed 

 and packed tightly together, giving to the snout the 

 rigidity which it requires. Nature thus supplies her 

 own need out of her existing resources, subjecting 

 them only to such modifications as each case 

 demands. Anyhow, we have here a first trace of 

 an introduction of membraniform bone amongst the 

 cartilaginous fishes. We next advance to those 

 fishes in which the skeleton consists of chondriform 

 and true membraniform bone developed side by side. 



