THE CRETACEOUS ‘PERIOD: 281 
the existing forms of the group in the fact that the bodies 
of the vertebra, like those of the Jurassic Crocodiles, are 
bi-concave, or hollowed out at both ends. In the Greensand 
of North America, however, occur the remains of Crocodiles 
which agree with all the living species in having the bodies of 
the vertebree in the region of the back hollowed out in front 
and convex behind. 
Birds have not hitherto been shown, with certainty, to have 
existed in Europe during the Cretaceous period, except in a 
few instances in which fragmentary remains belonging to this 
class have been discovered. The Cretaceous deposits of 
North America have, however, been shown by Professor 
Marsh to contain a considerable number of the remains of 
Birds, often in a state of excellent preservation. Some of 
these belong to Swimming or Wading Birds, differing in no 
point of special interest from modern birds of similar habits. 
Others, however, exhibit such extraordinary peculiarities that 
they merit more than a passing notice. One of the forms in 
question constitutes the genus /chthyornis of Marsh, the type- 
species of which (/. dispar) was about as large as a Pigeon. 
In two remarkable respects, this singular Bird differs from all 
known living members of the class. One of these respects 
concerns the jaws, both of which exhibit the Reptilian char- 
acter of being armed with numerous small pointed ‘éeeth (fig. 
212, a), sunk in distinct sockets. No existing bird possesses 
teeth; and this character forcibly recalls the Bird-like Ptero- 
saurs, with their toothed jaws. Jchthyornis, however, possessed 
fore-limbs constructed strictly on the type of the “wing” of the 
living Birds; and it cannot, therefore, be separated from this 
class. Another extraordinary peculiarity of /chthyornis is, that 
the bodies of the vertebre (fig. 212, c) were dz-concave, as 1s the 
case with many extinct Reptiles and almost all Fishes, but as 
does not occur in any living Bird. There can be little doubt 
that /c:hthyornis was aquatic in its habits, and that it lived prin- 
cipally upon fishes; but its powerful wings at the same time 
indicate that it was capable of prolonged flight. ‘The tail of 
Lchthyornis has, unfortunately, not been discovered ; and it is 
at present impossible to say whether this resembled the tail of 
existing Birds, or whether it was elongated and composed of 
separate vertebree, as in the Jurassic Archeopteryx. 
Still more wonderful than /chthyornis is the marvellous bird 
described by Marsh under the name of esperornis regalis. 
This presents us with a gigantic diving bird, somewhat re- 
sembling the existing ‘ Loons” (Colymbus), but agreeing 
with /chthyornts in having the jaws furnished with conical, 
