BONE. 581 



in the true bony scales comprising the exo-skeleton of the 

 cartilaginous fishes, the bone-cells are to be seen in great 

 numbers. In the spines of some of the Ray family may 

 be noticed a peculiar structure : the Haversian canals are 

 large and very numerous, and communicating with each 

 canal are an infinite number of wavy tubes, which are 

 connected with the canals in the same manner as the den- 

 tinal tubes of the teeth are connected with the pulp-cavity ; 

 and if such a specimen were placed by the side of a section 

 of the tooth of some of the Shark tribe, the discrimination 

 of one from the other would be no easy matter. In the 

 spine of a Kay, Fig. 322, the analogy between bone and 

 the ivory of the teeth is made more evident ; for in this 

 fish we have tubes, like those of ivory, anastomosing with 

 the canaliculi of bone-cells. 



" From our investigation of the minute structure of 

 the bone composing the skeleton in the four vertebrated 

 classes, let us proceed at once to the application of the 

 facts which have been laid down ; and let us, for example, 

 suppose that a fragment of bone of an extinct animal is 

 the subject of investigation. It has been stated, that the 

 bone-cells in Mammalia are tolerably uniform in size ; and 

 if we take 1 -2000th of an inch as a standard, the bone- 

 cells of birds will fall below that standard : but the bone- 

 cells of reptiles are very much larger than either of the 

 two preceding; and those of fishes are so entirely different 

 from all three, both in size and shape, that they are not 

 for a moment to be mistaken for one or the other ; so that 

 the determination of a minute yet characteristic fragment 

 of fishes' bone is a task easily performed. If the portion 

 of bone should not exhibit bone-cells, but present either 

 one or other of the characters mentioned in a preceding 

 paragraph, the task of discrimination will be as easy as 

 when the bone-cells exist. We have now the mammal, 

 the bird, and the reptile to deal with : in consequence of 

 the very great size of the cells and their canaliculi in the 

 reptile, a portion of bone of one of these animals can 

 readily be distinguished from that of a bird or a mammal ; 

 the only difficulty lies between these two last : but not- 

 withstanding that on a cursory glance the bone of a bird 

 appears very like that of a mammal, there are certain 



