Near the close of the Jurassic appeared for the first time, so far as we now know, broad-headed mesosuchian crocodiles, forms having less numerous and stronger teeth, and resembling closely modern alligators. It has been believed that these broad-headed kinds were of later origin than the more slender-nosed teleosaurs, but a moment’s consideration will make it evident how improbable such an evolution must be. The crocodiles must have descended from strictly terrestrial reptiles, and no terrestrial reptiles have a slender nose. That they should have acquired a slender face in adaptation to water habits and then returned to the more primitive land type with a broad face and less strictly aquatic habits is contrary to all our experience in paleontology. From this it is altogether probable that broad-faced crocodiles of later times must have been the descendants of broad-faced kinds that were in existence during all the Jurassic times, but of which we as yet have no knowledge. These broad-faced Jurassic crocodiles were, for the most part, small creatures, much smaller than the teleosaurs even, and smaller than any species of crocodiles now living. Their remains are known only from fresh-water or shore deposits, and are, for the most part, associated with those of land and fresh-water animals. About the time of their first known appearance in geological history, the small mammals and birds had also become more or less abundant, and it was suggested by Owen that these land animals had something to do with the development of the ancient amphibious crocodiles. Perhaps this was the case with respect to their greater abundance and development, and with certain peculiarities of their structure, but that the gavial-like teleosaurs should have come back to the land and reverted to a more primitive form seems quite improbable.