VERTEBRATE LIFE IN AMERICA. 527 



c 



evidently the parent stock of Crocodilians, became extinct with Hy- 

 posaurus of the same horizon, leaving the crocodile and gavial, with 

 their more perfect procoelian vertebras, to contend for the supremacy. 

 In the early Eocene, both of these types were abundant, but some of 

 the crocodiles possessed characters pointing toward the alligators, 

 which do not appear to have been completely differentiated until 

 later. 



Nothing is really known to-day of the earlier genealogy of the 

 Pterosauria ; but our American forms, without teeth, are clearly the 

 last stage in their development before this peculiar group became 

 extinct. The oldest European form, Dimorphodon, from the Lower 

 Lias, had the entire jaws armed with teeth, and was provided with a 

 long tail. The later genus, Pterodactylus, retained the teeth, but had 

 essentially lost the tail; while JRamphorhynchus had retained the 

 elongated tail, but had lost the teeth from the fore-part of both jaws. 

 In the genus Pteranodon, from the American Cretaceous, the teeth 

 are entirely absent, and the tail is a mere rudiment. In the gradual 

 loss of the teeth and tail, these reptiles followed the same path as 

 birds, and might thus seem to approach them, as many have supposed. 

 This resemblance, however, is only a superficial one, as a study of the 

 more important characters of the Pterodactyls shows that they are an 

 aberrant type of reptiles, totally off the line through which the birds 

 were developed. The announcement made not long since in Europe, 

 and accepted by some American authors, that the Pterosauria, in 

 consequence of certain points in their structure, were essentially birds, 

 is directly disproved by American specimens, far more perfect than 

 those on which the conclusion was based. 



It is now generally admitted, by biologists who have made a study 

 of the vertebrates, that birds have come down to us through the Dino- 

 saurs, and the close affinity of the latter with recent struthious birds 

 will hardly be questioned. The case amounts almost to a demonstra- 

 tion, if we compare, with Dinosaurs, their contemporaries, the Mesozoic 

 birds. The classes of Birds and Reptiles, as now living, are separated 

 by a gulf so profound that a few years since it was cited by the oppo- 

 nents of evolution as the most important break in the animal series, 

 and one which that doctrine could not bridge over. Since then, as 

 Huxley has clearly shown, this gap has been virtually filled by the 

 discovery of bird-like reptiles and reptilian birds. Compsognathus and 

 Archceopteryx of the Old World, and Ichthyomis and Hesperornis of 

 the New, are the stepping-stones by which the evolutionist of to-day 

 leads the doubting brother across the shallow remnant of the gulf 

 once thought impassable. 



[To be continued.'] 



