EVOLUTION OF BIKDS. 175 



classes and orders. The Labyrintkodonta, the most ancient of the 

 Amphibia, found as early as the carboniferous period, present many 

 piscine characters (ventral exoskeleton), and have a cartilaginous 

 skeleton. Many fossil orders and sub-orders of Saurians (Halo- 

 sauridce, Dinosauridce, PterodactyliduK (fig. 118), Tkecodontidcv) have 

 not left a single representative in the present day ; others again are 

 transitional between recent orders. Such a relation has, for example, 

 been recently shown between the " Pythonomorphous " lizards (related 

 to the genus Mosasaurus) from the chalk in America, and serpents 

 so far as the structure of the skull and jaw is concerned. 



Owen's researches on the fossil Reptiles of the Cape have shown 

 that certain Reptiles (Theriodontci) once lived there which showed 

 a close resemblance to carnivorous Mammalia with regard to their 

 dentition and the structure of their feet. The teeth of these 

 animals, though only furnished with one root, can be divided into 

 incisors, canine teeth, and molars, a fact which induces us to believe 

 it possible that the dentition of the most ancient Marsupials hitherto 

 known (Keuper) may be derived from that of a Theriodon-like 

 Reptile. 



Even as regards birds, a class so uniform in structure and so 

 sharply denned, a form (Archcvopteryx lithoyrapliica] (fig. 119) 

 transitional between them and Reptile has been discovered in the 

 Sohlenhofen slate, although the impression was not perfect. In this 

 form the short tail of the bird is replaced by a long reptilian tail 

 composed of numerous (20) vertebrae and provided with two rows 

 of feathers (Sawrurce). The articulation of the vertebral column 

 and the structure of the pelvis indicated an affinity to the long-tailed 

 Pterodactyls. 



The discovery of a second and more perfect specimen of Archceop- 

 teryx has made known to us its dentition. It had sharp-pointed 

 teeth wedged into the jaws. Other types of birds have also been 

 found in the American chalk, which diverge more widely among 

 theniseves and from the Saurians than do the birds of any living 

 order. These were defined as Odontornithes by Marsh,* and dis- 

 tinguished as a sub-class ; they had teeth in the jaws, which latter 

 were elongated to form a kind of beak. Some of them (Order 

 Ichthyornithes) had biccelous vertebrae, a crista sterni, and well 



* O. G. Marsh, " On a new sub-class of fossil Birds (Odontornithes)." 



Aiii/'rici/,// Jm/nifil of Sricncc and Art, Vol. v., 1873. 



O. C. Marsh, On the Odontornithes. or birds with teeth." Ani 

 Journal of Scicnec and Art, Vol. x.. 187ii. 



