55 



for the canaliculi which are derived from them being few in num- 

 ber: they are readily seen to anastomose freely with the canaliculi 

 given off from neighbouring cells ; and if the specimen under exami- 

 nation be a thin layer of bone, such as the scale of an osseous fish, 

 from the cells lying nearly all in one plane, the anastomosis of the 

 canaliculi will be rendered beautifully distinct. ]n the majority of 

 the bones of fishes the cells are not present, their place being occu- 

 pied (as has been before stated) by enlarged canaliculi or tubes ; but 

 in all the hard bones, the enlarged tubes and the cells together, or 

 else the cells alone, will be found. In the hard scales of many of the 

 osseous fishes, such as the Lepidosteus and Callicthys, and in the 

 spines of the Silurida, the bone-cells are very beautifully seen : in 

 the true bony scales composing 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 I have 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 con- 

 nected 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 be no easy mat- 

 ter. In the spine of a Ray 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. In 

 the scales, too, of the Dog-fish, and in a species of Shark, termed the 

 Squalus galens or tope, I have noticed a similar analogy to the dental 

 structure ; in the same animals the bone-cells are absent, but, as in 

 the dental structure, their place is supplied by ramifying canaliculi or 

 tubes. As a general rule, I may notice that in scales and other thin 

 plates of fishes' bone the cells are of a quadrate figure, but in the 

 bones composing the internal skeleton they are much more elongated, 

 like those of the Reptilia, but the cell is never so broad or so long, 

 neither are the canaliculi so numerous as those of the latter class. In 

 Plate vii. fig. 4, maybe seen some of the cells which approach nearest 

 those of reptiles, but still the eye will detect many differences be- 

 tween them and those in fig. 3. 



Having said thus much on the minute structure of the bone com- 

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



