LIFE IN SECONDARY TIMES 273 



Apart from the paired fins, it had a median fin on its back, 

 and its tail terminated in a fin divided into two unequal 

 lobes. That part of the vertebral column corresponding with 

 this fin described a sharp downward curve in the opposite 

 direction to that of heterocercal fishes. We saw (p. 230) that this 

 tail was an organ of levitation to bring them to the surface ; 

 that of the Ichthyosaurians, on the contrary, was a diving 

 organ. Lightened by the air in its lungs, the Ichthyosaurian 

 was naturally borne to the surface, and had to exert effort in 

 order to descend. 



Up to the present no transition of form has been found 

 between the Ichthyopterygians and terrestrial Reptiles, unless it 

 be Mixosaurus of the Trias, in which the radius and ulna are still 

 elongated and separated by a slight longitudinal interval. The 

 teeth, very numerous in Ichthyosaurians, became very small in 

 the Ophthalmosaurian of the English Jurassic and Cretaceous. 

 They have quite disappeared in Baptanodon of the Upper 

 Jurassic of Wyoming, as they have done in our Baleen-Whales. 



The Plesiosaurians are less isolated. They are linked with 

 Reptiles which, like themselves, had biconcave vertebrae, 

 and present an upper temporal fossa and ventral ribs, while 

 their limbs, still differing little from those of land Reptiles, 

 are, however, already adapted for swimming. These are the 

 Nothosaurians, primitive forms in which the notochord is pre- 

 served in the centum of the vertebrae. Mesosaurus, the initial 

 type of this group, is found in the Triassic sandstones of the 

 Karroo in the south of Africa and in those of Sao Paulo in 

 Brazil. They had only nine cervical vertebrae, and did not 

 exceed three decimetres in length. Lariosanrus attained a 

 length of one metre, and preserved its palatal teeth. Its neck 

 consisted of twenty vertebrae and its tail of forty, although these 

 were very short. Nothosaurus grew to a length of three metres 

 and had sixteen cervical vertebrae. Other forms have been found 

 in the Muschelkalk, near Magdeburg. In the Plesiosaurians, 

 properly so-called, which lived between the Lower Triassic 

 and the Jurassic, the neck was still more elongated, and 

 possessed as many as from twenty-eight to forty vertebrae. 

 The neck was longer yet in the Elasmosaurians, in which the 

 number of the vertebrae varied between thirty-five and seventy- 

 two. On the other hand, the tail was extremely short. The 

 Elasmosaurians differed especially from the Plesiosaurians in the 



