EVOLUTION OF EII5D9. "17-> 



classes and orders. The Ldbyrinthodonta, 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, Pterodactylidice (fig. 118), Thecodontidce) 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 (Theriodonta) once lived there which showed 

 a close resemblance to carnivorous Mammalia with regard to their 



o 



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

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

 incisors, canines, and molars, a fact which induces us to believe it 

 possible that the dentition of the most ancient Marsupials hitherto 

 known (Keuper) may have been derived from that of a Theriodon- 

 like Reptile. 



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

 sharply denned, a form (Archceopteryx lithographica) (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) vertebrse and provided with two rows 

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

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

 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 

 themseves 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 

 Ichthyornit-lces) had bicoelous vertebrae, a crista sterni, and well 



* 0. C. Marsh, " On a new sub-class of fossil Birds 

 American Journal of Science and Art, Vol. v., 187:-!. 



O. C. Marsh, " On the Odontornithes, or birds with teeth/' American. 

 Journal of Science and Art, Vol. x.. 1875. 



