20 PRESIDENT'S ADDRESS. 



Crocodiles (vertebrae concave on both faces) have no such 

 provision, as their food consisted of fish, or mammals which in 

 the Mesozoic Age were insignificantly diminutive. No Dinosaur, 

 Ichthyosaur, or Plesiosaur survived the Mesozoic Age. The 

 climatal changes through which the land and sea at that period 

 passed, caused their disappearance. 



The Ichthyosaurus and Plesiosaurus differ from each other both 

 in the size of the head and its connection with the vertebral 

 column. The head of Ichthyosaurus is large and like the Fish 

 and Whale joined to the trunk without an intermediate neck. 

 The head of the Plesiosaurus on the contrary is with a few 

 exceptions small and attached to a long neck. The eye of the 

 Ichthyosaurus was extremely large, in some cases the orbit is not 

 less than 16 inches. The eye-ball was protected by bony-scales 

 (sclerotic) articulated together. The vertebrae are biconcave like 

 fish. The cartilaginous union of the vertebrae gave them freedom 

 of movement, and their neural-arches connect them with the 

 highest of the cartilaginous Fishes. Like the Whale the cervical- 

 vertebrae are narrow and thin. The caudal-fin was vertical, in 

 which it differs from that of Fish, which is horizontal. The 

 clavicles form an arch, and unite in the central-line, resembling 

 the merry-thought of a bird. The sternum sends forth a lateral 

 prolongation on the upper portion of each side, corresponding 

 with the episternal pieces of the plastron of the Chelonia, 

 and approaching the scapular-arch of the Ornithorhyncus. The 

 fore-limbs are attached to the trunk by the scapular-arch. The 

 posterior limbs of the Plesiosaurus are much smaller than the 

 anterior ; their attachment to the trunk is by a feeble pelvic- 

 arch, they were used more as a rudder, than for progression. 



The iliac-bones are not connected with the sacral vertebrae, but 

 detached from the vertebral column, similar to the arch which 

 supports the ventral-fin of Fishes. The skin was naked like that 

 of the Whale. As its name implies the Plesiosaurus approached 

 more nearly the Saurian type than the Ichthyosaurus. This is 

 especially apparent in the conformation of the head, and the 

 reception of the teeth into distinct sockets, as with the Crocodile 



