200 PALEONTOLOGY 



abdominal cavity ; all of which characters have been demon- 

 strated by their fossil skeletons. With these characters the 

 sea-lizards combined the presence of two pairs of limbs shaped 

 like fins, and adapted for swimming. 



The group of reptiles so termed includes all those which 

 have any part of the thoracic-abdominal cavity encompassed 

 by moveable ribs. The first character distinguishes them 

 from the Batrachia and Chelonia with tin-shaped limbs. 



The Enaliosauria, however, do not form a strictly natural 

 group ; for this is based upon a single character relating to 

 merely the medium of life and locomotion. Some of the 

 labyrinthodont reptiles may have had their limbs in structure 

 and shape as paddles ; but more important modifications of 

 structure would keep them apart, like the lower Batrachia 

 and the Chelonia, from the more lizard-like reptiles called 

 EnalioMuria. 



In this group there are two divisions, — one characterized 

 by having five digits in the fin, the other by having more 

 than that typical number. The pentadactyle division may 

 be sub-divided into those in which the ilio-pubic arch is 

 attached to a sacrum and those on which it is freely sus- 

 pended or not so attached. The polydactyle division presents 

 a general type of structure more conformable with that of 

 which the Archegosaurs and Labyrinthodonts manifest two 

 phases of development, and in which the ascent from the 

 gano-salamandroid fishes reaches its culminating point in 

 Ichthyosaurus. 



Gemts Ichthyosaurus. — The name (from the Greek ichthys, 

 a fish, and sauros, a lizard) was devised to indicate the closer 

 affinity of the Iditliyosaur, as compared with the Plesiosaur, 

 to the class of fishes. The [chthyosaur (fig. 08) is remark- 

 able for the shortness of the neck and the equality of the 

 width of the back of the head with the fronl of the chest, 

 impressing the obsen er of the fossil skeleton with a conviction 



