FISH SCALES. 



721 



Fig. ?,5^.—Tra7isverse section of Horn 

 of Rhinoceros, seen by polarised 

 iisht. 



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

 of the canal bears a certain proportion to tlie size of the 

 bone-cells, and after soiue little ijractice the eye will 

 readily detect the difference. 



A curious modification of horn is presented in the ap- 

 pendage borne by the Ehinoceros upon its snout, which 

 in many points resembles a bundle of hairs. When a 

 transverse section is made and viewed by polarised liglit, 

 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). Whale- 

 bone is almost identical in 

 structure, and is similarly 

 affected by polarised light. See Plate Ylir. No. 178. 



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 

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

 which he names Flacoid, Ganoid^ Ctenoid, and Cycloid ; 

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



Fig. 354.— 6'caZe of Sole. 



3 A 



