^AUROPrERYGIA 



77 



knowledge of others of an intermediate kind between them and 

 land reptiles. His group-term then can be properly applied only 

 to the truly aquatic forms, and Owen's name Sauropterygia becomes 

 available in a wider sense to include all the known ty])es belonging 

 to the order of which the plesiosaurs form a part. Of this order 

 then there are two clearly marked divisions or suborders, the 

 Plesiosauria and the Nothosauria, the former having a complete 

 aquatic adaptation, the latter only a partial one. While the two 

 suborders are evidently allied, some authors have suggested that 

 their differences are only familial; others have thought that they 

 are really orders. We shall see how close the relationships are. 



PLESIOSAURIA 



It was Dean Buckland who facetiously likened the plesiosaurs 

 to a snake threaded through the shell of a turtle, and the simile 

 was not an inapt one in his day. The vernacular designation of 

 them — long-necked lizards — conveys the same impression of their 

 chief peculiarity, but the name is less applicable than it once was, 

 since recent discoveries have brought to light forms with a relatively 

 short neck. 



Though the plesiosaurs are nearly perfectly adapted to an 

 aquatic life, the adaptation was, in many respects, of a very differ- 

 ent kind from that of the ichthyosaurs — so very different that we 

 have not yet quite finished conjecturing as to the habits of the 

 living animals. As already suggested in the popular name, the 

 most striking characteristic of the typical plesiosaurs, the one which 

 suggested to Buckland his frequently quoted simile, is the ofttimes 

 enormously long neck, proportionately longer than that of any other 

 known creatures of the past or present. In other truly aquatic 

 animals the neck is actually shortened in the acquirement of a fish- 

 Hke shape, and the number of bones composing it reduced. In the 

 Sauropterygia the neck is usually longer than any truly land ani- 

 mals ever possessed, the longest-necked forms having as many as 

 seventy-six vertebrae in the cervical region. The elongation of the 

 neck among mammals is always due to an increase in the length of 

 the individual bones, never to an increase in the number from seven, 

 with but a single exception — a South American sloth which has 



