1914] Daniel: The Anatomy of Heterodontus francisci 149 



B. THE EXOSKBLETON, OR PLACOID SCALES 



The placoid scales of sharks form the outer protective covering 

 commonly known as shagreen. This in earlier times was in consid- 

 erable demand in the arts for polishing, but since the invention of 

 present-day sandpaper shagreen has been of little use. 



Barring the fact that placoid scales have occasionally been made 

 use of in systematic studies (see Helbing, 1902, and Klaatsch, 1890), 

 they have been of interest to the scientist principally because of the 

 structural similarity which they bear to teeth. Since the fundamental 

 work of Hertwig (1874), in which they were demonstrated to be 

 essentially identical with vertebrate teeth, placoid scales have called 

 forth little study. But that they possess an intrinsic interest can, I 

 think, be demonstrated. 



Among present-day elasmobranchs I know of no type the scales of 

 which exceed in beauty and in complexity of form those of the hetero- 

 dont sharks. In the study which I have made I have been particu- 

 larly fortunate in having an almost complete series from embryo to 

 adult. In this series I have studied with increasing interest the 

 building up of this complexity from a comparatively simple beginning, 

 and its assumption of the beauty characteristic of the adult scales. 



I. The Preparation of Scales for Study 



In the preparation of scales for study several methods have been 

 employed. In some cases bits of the integument were soaked in glyce- 

 rine for from twenty-four to forty-eight hours in order to clear the 

 tissue surrounding the scales. Preparations of this sort have been 

 found valuable chiefly in the study of patterns. In the majority of 

 studies which I have made, pieces of the integument have been kept 

 a day or two in a 10 per cent solution of sodium hydroxide. In this 

 the tissue around the neck and base of the scale is eaten out so that 

 the scale can be studied in detail. In the third plan which I have 

 used, after the scales had been soaked in sodium hydroxide they were 

 dissected out and mounted in thickened balsam. All of the scales not 

 figured in plates in the present paper were prepared in the third way. 



In addition it may be said that the scales in all of the patterns of 

 the plates were drawn to the same magnification by the aid of the 

 camera lucida. 



