PROCURING OBJECTS. 93 



Enamelled Scales. 



1. Placoidians. Cartilaginous fishes, having prickly or flat- 

 tened spines, as the skates, dog-fish, and sharks. 



2. Gandidians. With angular scales composed of horny or 

 bony plates covered with enamel, as the sturgeon, and bony 

 pike. Fifty out of sixty genera are extinct. 



Scales not Enamelled. 



3. Cteno'idians. Scales notched or serrated on their posterior 

 free edges, as the perch. 



4. Cycloid fishes, with smooth scales, more or less circular, 

 and laminated, as the herring, salmon, &c. 



Among the various kinds of fish-scales selected for micro- 

 scopic objects, those of the eel are much prized, as it was for- 

 merly considered that it had no scales. They may be obtained 

 from the under surface of the skin with a knife or a pair of 

 forceps. 



Some scales when viewed by polarized light have a brilliant 

 effect. They may be mounted in balsam. Fossil scales, as well 

 as others, may be examined as opaque objects. 



HAIR or ANIMALS, ETC. Hairs are composed of an aggre- 

 gation of epithelium cells, and the color depends upon the 

 quantity of pigment deposited in or about each cell. They may 

 therefore be called elongated developments of the epidermis. A 

 transverse section is not always round, but may be oval, flat- 

 tened, or reniform. Henle has shown that the curling of hair 

 depends upon its form, and that the flatter the hair the 

 more it curls, the flat side being directed towards the curve 

 described. P. A. Browne, Esq., of Philadelphia, has attempted 

 to show a specific difference in the races of men from the shape 

 of the transverse sections of their hair, but we think without 

 success. It is not likely that scientific investigation will ever 



