720 TKS MICROSCOPE 



to be seen in great numbers. In the spines of some of the 

 Eay family may be noticed a peculiar structure : the Haver- 

 sian canals are large and very numerous, and communi- 

 cating with each canal are an infinite number of wavy 

 tubes, which are connected with the canals in the same 

 manner as the dentinal 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 bo 

 no easy matter. In the spine of a Eay, fig. 352, 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. 



Now, if we proceed at once to the application of the 

 facts which have been laid down, and make a fragment of 

 bone of an extinct animal the subject of investigation ; 

 we first find that the bone-cells in Mammalia are tolerably 

 uniform in size ; and if we take 1 -2,000th 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 discrimi- 

 nation 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, notwithstanding that on a cursory glance 

 the bone of a bird appears very like that of a mammal, 

 there are certain points in their minute structure in which 

 they differ ; and one of these points is in the difference ir 

 size of their bone-cells. To determine accurately, there- 

 fore, between the two, we must, if the section be a trans- 

 Verse one, also note the comparative sizes of the Haver?iap 



