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bone-cells in Mammalia are tolerably uniform in size, and if we take 

 -rarvth 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 mi- 

 nute yet characteristic fragment of fishes' bone is a task easily per- 

 formed. 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 discrimimation 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 ani- 

 mals 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 in size in their 

 bone-cells. I have stated that the average size of the cells in Mam- 

 malia is -j-cTj-tfth of an inch, whilst in birds they are much smaller : to 

 determine accurately, therefore, between the two, we must, if the sec- 

 tion be a transverse one, also note the comparative sizes of the Ha- 

 versian canals, and the tortuosity of their course, for the diameter of 

 the canal bears a certain proportion to the size of the bone-cells, and 

 after some little practice the eye will readily detect the difference. 

 The fragments necessary for the purpose of examination are to be se- 

 lected with some little care, and on the whole a small chip (or two) 

 from the exterior of the shaft of a long bone is sufficient ; but as 

 many fossil bones are coated with a layer of earthy deposit on their 

 external surface, it will be requisite to get beneath this deposit, as it 

 very seldom happens that the bone-cells are visible in it, but by a 

 fragment from about the middle of the laminae of the shaft the charac- 

 teristic bone-cells can at once be recognised. But in the comparison 

 of the bone of a mammal with that of a bird, I prefer transverse sec- 

 tions, as from the peculiarity in the arrangement of the Haversian ca- 

 nals in the latter class, it is highly important always to bear in mind 

 that the specimens used for comparison should be cut in one and the 

 same direction, for as it has been stated that the bone-cells, on which 

 we are to rely for our determination, are always longest in the di- 

 rection of the shaft of the bone, it would follow, that if one section 

 were transverse, and the other longitudinal, there might be a vast 



