Page:Popular Science Monthly Volume 73.djvu/563

Rh little more than tantalize the student with the hints of new forms and new relationships which can not be verified. In some places these water-worn fragments are so thick upon the ground that they can literally be shoveled up by the wagon load. In some rare instances the bodies of animals found their way unharmed into the water and, distended by gases of decomposition, floated far and uninjured until they came to rest on some mud flat beyond the reach of sharks or other predatory animals. Such skeletons are preserved entire and in a most wonderful state of preservation, but they are exceedingly rare, not more than one turning up in a season's search.

Of all the wonderful animals revealed by their petrified remains, perhaps the most striking are the reptiles. The reptiles made their first appearance in the Permian or in the latter portion of the Carboniferous preceding, but here at the very inception of their line they developed a great diversity of form and habit. There were aquatic and terrestrial forms, carnivorous, herbivorous and omnivorous forms, forms simple and closely resembling their amphibian ancestors and forms so bizarre in their structure that the world has produced nothing more strange. The reptiles descended from the amphibians and it is natural to look in these beds, where the lowest of the reptiles are found, for the connecting link between the two, but as yet this form has not been discovered; the approach from both sides, however, is so close that it is frequently impossible to determine the nature of a specimen from a single bone or small portion of the skeleton.

The simplest of the early carnivorous reptiles were aquatic, living in the waters of the great rivers or perhaps even in the ocean. The body was long and slender and" the tail was exceptionally so, in correlation with the swimming habit. Aside from the more technical points, the interest in the development of the primitive reptiles centers in certain changes of the teeth and the dorsal spines of the vertebra?. In one of the simplest forms, Poliosaurus, the teeth have the form of simple cones of nearly equal size in all parts of the jaw; such a dentition indicates that it preyed upon small animals which it seized and swallowed whole after the manner of snakes and many lizards. The dorsal spines are low and do not project beyond the skin. The animal probably resembled very closely the living monitor of the Nile.

In another and closely related reptile, Theropleura, the teeth have become differentiated, those at the anterior end of both the upper and the lower jaws, the incisors, are enlarged and have taken on the appearance of tusks; posterior to the incisors there is a slight notch in the edge of the upper jaw, caused by the growth of the lower incisors, and posterior to this there are tusk-like teeth in the upper jaw, the canines. The growth of the incisors and canines increased the power of grasping and holding prey. Moreover, in the posterior teeth of the