272 Marsh’s Monograph on the Odontornithes 
: ip 
the highly specialized feature of teeth in distinct sockets, 
Better examples than these could hardly be found to illustrate 
one fact brought out by modern science, that an animal may 
attain great development in one set of characters, and at the 
same time retain other low features of the ancestral type. This 
is a fundamental principle of evolution. 
“The more superficial characters of the absence of wings 
and the strong swimming legs and feet of J/esperornis are in 
striking contrast, also, with the powerful wings and diminutive 
legs and feet of Jchthyornis. These and other characters, 
already mentioned, separate the two birds so widely that a 
more detailed comparison seems here unnecessary. 
“Tt would be highly desirable to carefully compare both 
Ichthyornis and Hesperornis with Archwopteryx, the still older 
Mesozoic bird. This unfortunately cannot be done at present, 
as the two skeletons of Archeopteryx, now known, have not yet 
been fully described, nor even prepared for examination by 
bs The other Mesozoic birds now known from the deposits of 
this country, and the few discovered in Europe, may, some or 
all of them, have had teeth, but their remains are too fragmen- 
tary to determine this point, or even their near affinities. 
“Tt is an interesting fact that the Cretaceous birds at present 
known, some twenty species or more, were all apparently a A 
tic forms, which of course are most likely to be preserved 1? 
marine deposits, while the Jurassic Archaeopteryx, the only one 
from that formation, was a true land bird. 
“The birds found in more recent formations all belong app 
rently to modern types, and hence present few points for pro- 
fitable comparison with the Odontornithes. The existing birds 
with reptilian characters are nearly all confined to the Rat, 
