220 PALEONTOLOGY. 



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

 liked fins, and adapted for swimming. The thoracic-abdominal 

 cavity being encompassed by movable ribs, distinguishes 

 them from the Batrachia and Chelonia with fin-shaped limbs. 



The Enaliosauria, being associated chiefly by the loco- 

 motive organs relating to the medium, of life, admit of sub- 

 division into two orders by the modifications of those organs. 

 One is characterized by having five digits in the fin, the other 

 by having more than that typical number. The pentadactyle 

 division may be subdivided into those in which the ilio-pubic 

 arch is attached to a sacrum and those on which it is freely 

 suspended or not so attached. The polydactyle division pre- 

 sents 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. 



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

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

 affinity of the Ichthyosaur, as compared with the Plesiosaur, to 

 the class of fishes. The Ichthyosaur (fig. 89) is remarkable for 

 the shortness of the neck and the equality of the width of the 

 back of the head with the front of the chest, impressing 

 the observer of the fossil skeleton with a conviction that the 

 ancient animal must have resembled the whale tribe and the 

 fishes, in the absence of any intervening constriction or neck. 



This close approximation in the Ichthyosaurs to the form of 

 the most strictly aquatic vertebrate animals of the existing 

 creation, is accompanied by an unusual number and shortness 

 of the vertebrae, as in sharks ; and by a similar modification 

 of the surfaces forming the joints of the back-bone, each of 

 which surfaces are hollow, leading to the inference that they 

 were originally connected together by an elastic bag or " cap- 

 sule" filled with fluid — a structure which prevails in the class 



