5 26 THE POPULAR SCIENCE MONTHLY. 



In this brief summary of the past life of reptiles and birds in Amer- 

 ica, I have endeavored to exclude doubtful forms, and those very im- 

 perfectly known, preferring to present the conclusions reached by 

 careful study, incomplete though they be, rather than weary you with 

 a descriptive catalogue of all the fossils to which names have been 

 applied. Even this condensed review can hardly fail to give you 

 some conception of the wealth of our continent in the extinct forms 

 of these groups, and thus to suggest what its actual life must have 

 been. 



Although the Trias offers at present the first unquestioned evidence 

 of true reptiles, we certainly should not be justified in supposing for 

 a moment that older forms did not exist. So too in considering the 

 different groups of reptiles, which seem to make their first appearance 

 at certain horizons, flourish for a time, and then decline, or disappear, 

 every day brings evidence to show that they are but fragments of the 

 unraveled strands which converge in the past to form the mystic cord 

 uniting all life. If the attempt is made to follow back any single 

 thread, and thus trace the lineage of a group, Ave are met by difficul- 

 ties which the science of to-day can only partially remove. And yet 

 the anatomist constantly sees in the fragments which he studies 

 hints of relationship which are to him sure prophecies of future dis- 

 coveries. 



The genealogy of the Chelonia is at present unknown, and our 

 American extinct forms, so far as we now have them, throw little 

 light on their ancestry. This is essentially true, also, of our Plesio- 

 sauria, JOacertilia, and Ophidia, although suggestive facts are not 

 wanting to indicate possible lines of descent. With the Crocodilia, 

 however, the case seems to be different, and Huxley has clearly pointed 

 out the path for investigation. It is probable that material already 

 exists in our museums for tracing the group through several impoi'tant 

 steps in its development. We have already seen that the modern 

 proccelian type of this oi-der goes back only to the Upper Cretaceous, 

 while the Eelodonts, of our Triassic rocks, with their biconcave verte- 

 brae, are the oldest known Crocodilians. Our Jurassic, unfortunately, 

 throws but little light on the intermediate forms, but we know that 

 the line was continued, as it was in the Old World through Teleosaiirus. 

 The beds of the Rocky Mountain Wealden ' have just furnished us 

 with a genuine " missing link," a saurian (Dlplosaurus) with essen- 

 tially the skull and teeth of a modern crocodile, and the vertebra? of 

 its predecessor from the Trias. This peculiar reptile clearly represents 

 an important stage in the progressive series, and evidently one soon 

 after the separation of the crocodile branch from the main stem. The 

 modern Gavial type appears to have been developed about the same 

 time, as the form was well established in the Upper Cretaceous genus, 

 Thoracosaurus. The Teleosaurian group, with biconcave vertebrae, 



1 See note on page 520, also section. 



