The original drawings, made especially for this book, are by Charles R. Knight and James M. Gleeson, under the direction of Mr. Knight. The fact that the originals of these drawings have been presented to and accepted by the United States National Museum is evidence of their scientific value. Mr. Knight has been commissioned by the Smithsonian Institution, the United States National Museum, and the New York Museum of Natural History, to do their most important pictures of extinct animals. He is the one modern artist who can picture prehistoric animals with artistic charm of presentation as well as with full scientific accuracy. In this instance, the author has personally superintended the artist's work, so that it is as correct in every respect as present knowledge makes possible. Of the minor illustrations, some are by Mr. Bruce Horsfall, an artist attached to the staff of the New York Museum of Natural History, and all have been drawn with the help of and under the author's supervision.