170 GEOLOGICAL DISTKIBUTION. 



the jaws appear to have been encased in horn, and to have been 

 entirely edentulous. 



Bird remains are sufficiently abundant in certain localities, many 

 of them belonging to forms seemingly not very far removed from 

 some of our modern groups. But, in addition to these ordinary 

 forms, we have some of the most extraordinary of any that have ever 

 been described, and which, from the presence of true teeth in their 

 jaws, have received the name of Odontornithes (toothed-birds). In 

 the genus Ichthyornis, as exemplified in I. dispar, which was of 

 about the size of a pigeon, in addition to the peculiarity of alve- 

 olar teeth that of biconcave vertebrse is presented, a structure of 

 the vertebral column characteristic of fishes and many of the ex- 

 tinct reptiles, but not known in modem birds. The wings ap- 

 pear to have been well developed, and in this, and all other respects 

 beyond those just mentioned, the animal conformed strictly to the 

 modern type of bird structure. In the still more remarkable Hes- 

 perornis, which in the species H. regalis attained a height of five 

 or six feet, the teeth, instead of being implanted in distinct sockets, 

 were placed in a continuous groove ; the extremity of the upper jaw 

 appears to have been bent down in the form of a beak, and to have 

 been edentulous. The breastplate was entirely destitute of a keel 

 or ridge for the attachment of the powerful muscles required for the 

 motion of the wings, so that the bird was doubtless completely de- 

 nied the power of ffight. The presence, in the same geological 

 period and the same geographical area (Kansas), of two birds so 

 closely related to each other in the presence of jaw-teeth, and yet 

 so distantly removed from each other by other peculiarities of 

 stnicture, argues strongly for the antiquity of this class of animals, 

 and, though the earliest unequivocal traces of birds have thus far 

 been met with in the deposits of the Jurassic period, it is more than 

 probable that their first origin is considerably mpre ancient. 



No traces of any mammalian have thus far been discovered in 

 any indisputably Cretaceous deposit, a circumstance in great part 

 attributable to the particular conditions under which most of the 

 deposits of this period, as known to us, were laid down, namely, 

 their marine origin. But there can be no doubt that at some future 

 day such remains will be found, and, indeed, if the deposits of the 

 Laramie age be conceded to be absolutely Cretaceous, as is claimed 

 (although on most contradictory evidence) by many geologists, then 



