666 THE AMERICAN NATURALIST. [Vou. XXXII. 
straight forms assumed by certain of the ammonites when on 
the verge of extinction were likewise cases of convergence, and 
due to weakness or senility, or at least to an unusual and 
unfavorable condition of the seas in which they lived. 
The physical causes of extinction of the Mesozoic reptiles 
may also have been due to, or connected with, the changes of 
coast level, although signs of weakness and senility are exhib- 
ited by these. In the Como or Atlantosaurus beds referred 
by Scott to the Lower Cretaceous rather than Jurassic, the 
ichthyosaur (Sauranodon natans) was toothless, while the colos- 
sal Cretaceous pterodactyle Ornithostoma (Pteranodon) was 
entirely toothless. 
The colossal Pythonomorpha, offshoots of terrestrial lizards, 
but with paddles adapting them for marine existence, succeeded 
the plesiosaurs, and may have materially aided in their extinc- 
tion. Hence arises the question, Did the extinction of the 
marine reptiles result in, or contribute to, the great increase of 
teleost fishes ? 
Before the dinosaurs began to die out, the type in part 
became specialized into lizard-like, tree-climbing forms, and 
agile, bird-like forms. The first birds of the Cretaceous were 
toothed, carinate, highly predaceous forms, with a retrogressive 
side branch of wingless diving birds, represented by the colossal 
Hesperornis, but in this case the loss of teeth was undoubtedly 
a gain to the type, compensation for the lack of a dental arma- 
ture in the seed-eating birds being shown in the elaboration 
of a gizzard. 
5. Geological Changes in the Tertiary.— Here again we 
have, as in former periods, a succession of earth movements, 
subsidences in one region and elevations in another, though 
apparently more limited in extent than before, the oscillatory 
movements being rather confined to coastal areas, and involv- 
ing adjacent shallow seas, there being frequent alternations 
of marine with brackish and fresh-water beds. As Kayser 
remarks, the Tertiary deposits “no longer extended unaltered 
over whole countries like those of older systems, but generally 
occupied only smaller basin or bay-like areas, filled-up inland 
seas or shallow gulfs” (p. 328). Towards the close of the 
