A great part of the river systems of the country with their flooded areas are highly favourable to the development of reptilian life.
Jurassic SystemThis system is not known with certainty in the eastern half of the United States, though there are some beds on the mid-Atlantic coast, along the inland border of the coastal plain, which have been thought by some, on the basis of their reptilian fossils, to be Jurassic. The lower and middle parts of the system are but doubtfully represented in the western interior.
The Eye is essentially reptilian, but in sharpness of vision, power and quickness of accommodation it surpasses that of the mammals.
Reptilian remains abound; plants are also plentiful.
The fate of the reptilian quadrate bone (which is reduced to very small dimensions in the Anomodontia) has been referred to in an earlier section of the present article, where some mention has also been made of the disappearance in mammals of the hinder elements of the reptilian lower jaw, so as to leave the single bone (dentary) of each half of this part of the skeleton in mammals.
More dimly still visions of what the first bird may have been like could be reasonably entertained; and, passing even to a higher antiquity, the reptilian parent whence all birds have sprung was brought within reach of man's consciousness.
The separation of the Ratitae from the other birds, and their seemingly fundamental differences, notably the absence of the keel and of the power of flight, induced certain authors to go so far as to derive the Ratitae from the Dinosaurian reptiles, whilst Archaeopteryx (q.v.) and the Carinatae were supposed to have sprung from some Pterosaurian or similar reptilian stock.
Fossil reptilian remains, chiefly Dicynodon, are abundant.
- Representative Mammalian, Avian and Reptilian Trypanosomes, to illustrate the chief morphological characters.
In the western interior there is difference of opinion as to whether certain beds rich in reptilian remains (the Morrison, Atlantosaurus, Como, &c.) should be regarded as Jurassic or Comanchean.
To this division are assigned the yellow sandstones of Elgin, which have yielded crocodilian and other reptilian remains, the discovery of which led to the rocks being separated from the Upper Old Red Sandstone, to which they had previously been thought to belong.
Archaeopteryx was a bird, without any doubt, but still with so many low, essentially reptilian characters that it forms a link between these two classes.
Obvious peculiarity is the long reptilian tail, composed of 20 vertebrae and not ending in a pygostyle.
The whole wing is consequently, although essentially avine, still reptilian in the unfused state of the metacarpals and the numbers of the phalanges.
An osteological question which has been much discussed is the fate of the reptilian quadrate bone in the mammalian skull.
The tympanum itself has been regarded as representing one of the elements - probably the supra-angular - of the compound reptilian lower jaw.
This does not mean that any of the living groups of reptiles can claim their honour of ancestry, but it means that the mammals have branched where the principal reptilian groups meet, and that is a long way back.
Osborn, the tripartite reptilian condyle, by the loss of its median element, has given rise to the paired mammalian condyles; so that this difficulty disappears.
Others however (inclusive of Tritylodon and Microlestes, if they be really mammals), seem nearer to the Monotremata; and the question has yet to be decided whether placentals and marsupials on the one hand, and monotremes on the other are not independently derived from reptilian ancestors.
At any rate, there seems little doubt that it was the region where creodonts and other primitive mammals were first differentiated from their reptilian ancestors.
Dromotheriidae, and apparently showing decided traces of reptilian affinity.
The muscles of the limbs show a great amount of specialization, away from the fundamental reptilian and mammalian conditions.
These contain reptilian remains, and also clear imprints of leaves of the Glossopteris indica, as well as other indications of an ancient vegetation.
The lizards and snakes are the two dominant reptilian orders which are still on the increase in species, though certainly not in size.
The reptilian fauna exhibits an exceptionally large number of interesting genera and species.
- The more characteristic features of the bird's brain show clearly a further development of the reptilian type, not always terminal features in a direct line, but rather side-departures, sometimes even a secondary sinking to a lower level, and in almost every case in a direction away from those fundamentally reptilian lines which have led to the characters typical of, and peculiar to, the mammals.
The right and left halves are completely divided by septa, no mixture of the venous and arterial blood being possible, an advance upon reptilian conditions, even the highest.