One of the most interesting and most vexed questions in vertebrate palæontology at the present day is the one which concerns the origin and development of the reptiles. The progress of research has shown the Reptilia to be a wonderfully diverse class, and nearly thirty orders of animals have been assigned to the group. With all this diversity of structure there is, of course, associated a great diversity of habit. We know all kinds of extinct reptiles from the semi-aquatic to the aquatic, from the flying to the fossorial, from the arboreal to the subterranean, and many are the varying degrees of structure which are associated with these various habits of life.