FISH SCALES. 



721 



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. 



A curious modification of horn is presented in the ap- 

 pendage borne by the Ehinoceros u-pon its snout, which 

 in many points resembles a bundle of hairs. When a 

 transverse section is made and viewed by polarised light, 

 each cylinder is seen to have 

 a cross diverging from a cen- 

 tral spot; the lights and 

 shadows of this cross are 

 replaced by bands of con- 

 trasted complementary co- 

 lours, if the selenite plate is 

 interposed (fig. 353). See 



Pifltp VTTT ~NTn T78 Whal P Fi S- 953. Transverse section of Horn 



nate v 111. IN o. i / s. w naie- of RUnoceroS) seen by polarised 



bone is almost identical in light. 



structure, and is similarly affected by polarised light. 



A knowledge of the form and structure of scales of 

 fishes (fig. 354), like that of 

 teeth, has been "shown by 

 M. Agassiz to afford an uner- 

 ring indication of the particu- 

 lar class to which the fish may 

 belong : in the examination 

 of fossil remains, the appli- 

 cation of this knowledge has 

 been attended with extraor- 

 dinary results. As a class of 

 objects for the microscope, the 

 scales of fishes are exceedingly 

 curious and beautiful, especially 

 when mounted in fluid or 

 Canada balsam, and viewed by 

 polarised light. Many are seen 

 best as opaque objects, and are 



then mounted dry between Fig. 354ScaZe o/ sofc. 

 glasses. M. Agassiz divides the scales into four orders, 

 which he names Placoid, Ganoid, Ctenoid, and Cycloid ; 

 in the first two the scales are more or less coated with 

 3 A 



