1. Hoceboom, F.E., Practical Pedodontia or Juvenile Operative Dentistry and Public Health Dentistry , 4th Ed. (St. Louis , C. V. Mosby Company: 1938), p. 132.
2. McBride, W.C., Juvenile Dentistry, 2nd Ed. ( Philadelphia, Lea & Febiger: 1937), p. 230.
3. Angle, E.H., Treatment of Malocclusion of the Teeth, 7th Ed. (Philadelphia, S. S. White Dental Manufacturing Company: 1907), p. 19.