554 HISTORICAL GEOLOGY 



the contents of the abdomen, which often contains fish scales and the 

 remains of shelled cephalopods (belemnites, p. 531). 



They were remarkably well adapted to aquatic life, as is shown by 

 the paddle-like limbs ; by the outline of the body, which was so modi- 

 fied as to permit movement through the water with as little resistance 

 as possible; by the sharp teeth for the catching and retention of 

 slippery prey. The occurrence of undigested, immature young 

 within the ribs of a number of specimens indicates that their offspring 

 were produced alive. 



Although only the later stages of the evolution of the ichthyosaurs 

 are known, yet it is evident that they were descended from land 

 reptiles. This is shown by the structure of the limbs of the earlier 

 forms, which were more like the legs of land animals than were those 

 of the later species. 



The following progressive changes, fitting the animal for marine 

 existence, have been traced: (1) The limb became more paddle-like 

 and less leg-like, both in the structure of the skeleton and in the 

 external shape. (2) The head became longer and better adapted 

 for catching fish and other slippery animals. (3) The eyes became 

 larger and more efficient for seeing in the water. (4) The neck be- 

 came shorter. (5) The body gradually became more fishlike in 

 shape and could move through the water more rapidly and with less 

 resistance. 



Ichthyosaurs began in the Triassic, culminated in the Jurassic, 

 and lived, for a short time, in the Upper Cretaceous. During the 

 Jurassic they appear to have been very abundant and to have oc- 

 cupied every sea. 



Plesiosaurus (Greek, plesios, near, and saurus, reptile). — These 

 marine reptiles are characterized (Fig. 515) by a short, stout body, 

 a short tail, and usually by a long neck and small head. The tail 

 was probably of greater use in steering than as an organ of propulsion, 

 the powerful, paddle-like limbs being for that purpose. These pad- 

 dles had five digits, but each digit was made up of a large number 

 of small bones, in some cases as many as 20. Plesiosaurs varied 

 greatly in size, some being 30 to 40 feet long, but they usually did not 

 attain a greater length than 6 to 15 feet. One American species 

 (Elasmosaurus), for example, was 40 feet long, with a small head and a 

 neck 22 feet in length. "The other extreme was Pliosaurus, equally 

 huge in bulk, but with a skull nearly q feet long and a neck of only a 

 foot and a half." Most of the smaller Plesiosaurs had small heads. 



