486 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxii. 



maxilla, strongly ridged and anisodont. Parietals rounded and 

 obtuse above, not elevated into a crest. Temporal vacuities large; 

 zygomatic bars slender posteriorly. Pterygoids broadly united in 

 the middle behind ; the interpterygoidal vacuities short, at the bottom 

 of a depressed pit. No palatine foramina. Palatines broadly united 

 in the middle in front of the pterygoids. Cervical vertebrae 1.3 in 

 number, smootlily rounded below, without vascular foramina, shal- 

 lowly concave at extremities; cervical vertebrae broader than long; 

 cervical ribs single-headed. Benton Cretaceous of Kansas and Texas. 

 Relationships of the Plesiosaurs. — In the attempt to reach some 

 defuiite conclusions as to the habits of the plesiosaurs, I gave ^ five 

 years ago the following list of adaptive characters in aquatic, air- 

 breathing vertebrates : 



1. Elongation of the head, with attenuation of the facial region. 



2. Elongation of trunk and tail, but especially the latter, with pro- 

 gressive weakening of the zygapophysial articulations posteriorly. 



3. Shortening of the neck. 



4. The acquirement of a caudal fin. 



5. The acquirement of sclerotic plates. 



6. Recession of the external nares. 



7. Absence of the sacrum and the absence or progressive obsoles- 

 cence of the sternum. 



8. Greater slenderness and smaller size or loss of the hind limbs. 



9. H3q^erphalangy and hyperdactyly. 



10. Smoothness of the skin. 



11. Sponginess of the bones of the skeleton. 



12. Increase in number and decrease in size of teeth. 



The exceptions which the plesiosaurs present to these adaptations 

 are : 



1. Elongation of the neck, with increase in number of vertebrae. 



2. Shortening of tail and body, and the flattened, depressed form 

 of the latter. 



3. The presence of a well-defined sacrum of three vertebrae. 



4. The somewhat greater slenderness of the hind limbs, but with 

 little or lio decrease in effectiveness as propelling organs. 



In these exceptions the plesiosaurs agree with the marine turtles. 

 In the tail-propelling, aquatic vertebrate the propodial bones are 

 invariably shortened, as for instance in the Cetacea, Ichthyosauria, 

 Pythonomorpha, and the front legs of the Thalattosuchia, and the 

 limbs become merely equilibrational organs in direct proportion to 

 the effectiveness of the tail as a propelling organ. Experiments on 

 fishes show that the loss of the paired fins does not impau* the swim- 

 ming powers of the individual, but does require the constant vibra- 

 tional use of the tail in the preservation of the equilibrium, while the 



a Kansas University Science Bulletin, I, 1902, p. 259. . 



