TECHNICAL NOTES AND BIBLIOGRAPHY 247 



68. Smith, H. W. The absorption and excretion of water 

 and salts by the elasmobranch fishes. 11. Marine 

 elasmobranchs. American Journal of Physiology, 98: 

 296. 1931. 



69. Smith, H. W. The retention and physiological role 

 of the urea in the Elasmobranchii. Biological Re- 

 views, 11: 49. 1936. 



70. Smith, H. W., and C. G. Smith. The absorption 

 and excretion of water and salts by the elasmobranch 

 fishes. I. Fresh- water elasmobranchs. American Jour- 

 nal of Physiology, 98: 279. 1931. 



71. White, E. G. A classification and phylogeny of the 

 elasmobranch fishes. American Museum Novitiates, 

 No. 837, April, 1936. 



72. White, E. G. Interrelationships of the elasmo- 

 branchs with a key to the order Galea. Bulletin of 

 the American Museum of Natural History, 74: II, 

 p. 25. 1937. 



73. Young, L. Z. The Life of Vertebrates. Clarendon 

 Press, Oxford, 1950. 



In the surviving marine remnants of the ostracoderms, 

 the hagfishes (Myxine) and lampreys (Petromyzon), 

 the blood is essentially isosmotic with sea water; either 

 the ancestral forms assumed a salt-water habitat before 

 mechanisms were available for controlling the composi- 

 tion of the body fluids in salt water, or else out of 400- 

 odd million years natural selection has left us only two 

 remnants which gave up the fight and took the osmoti- 

 cally easiest way {47, 65, p. 119}. Our classification of 

 the ostracoderms foUows that of Gregory {33} and 

 Romer {41, 42}. The evolution of fins from the spines 

 of the ostracoderms is discussed by Gregory and Raven 

 {34}. 



The cartilaginous fishes (Chondrichthyes) are divided 

 by some writers into two subclasses, the Elasmobranchii 

 and the Holocephali (Chimaerae) {42}, while others 

 {49» 73} include both in the class Elasmobranchii. The 



