Johnson — Axial Bifurcation in Snalces. 535 



SUMMARY AIS^D COK'CLUSION". 



1. Thirteen two-headed snakes are described by means of 

 skiagraphs. 



2. The previous descriptions of two-headed snakes are re- 

 viewed. 



3. It is conchided that this abnormality is more abundant 

 in some species than in others and that the point of bifurca- 

 tion is most likely to occur in the cephalic half of the snake, 

 between 6 a.nd 13^ of the entire length from the cephalic end. 



4. The point of bifurcation of the vertebrae is more posterior 

 than would be supposed from external examination. The 

 skulls frequently appear united externally when in reality 

 thev are not. 



I am much indebted to Prof. C B. Davenport for kind ad- 

 vice and assistance, to Mr. Samuel Garman for help in deter- 

 mination of species, to Mr. Outram Bangs, Mr. F. K. Mixer, 

 Mr. J. F. Whiteaves, Prof. Edwin Linton, Mr. F. W. True 

 and to Prof. W. E. Bitter for loans of material, and to Dr. W. 

 McM. Woodworth for assistance with the bibliography and 

 for the loan of books. 



Anatovfiical Lahoraiory, University of ^Yisconsil^, 

 November 21, 1901. 



BIBLIOGBAPHY. 



Aldrovandi, Ulyssis. 1640. Historise Serpen tmn et Dra- 

 conum, Libri Duo, Bononiss. p. 60. 



Bancroft, Edward. 1769. Natural History of Guiana. 

 London, p. 214. 



Bateson, Wm. Materials for the Study of Variation. Lon- 

 don. 1894. p. 561. 



Boettger, O. 1890. Ber. lib d. Senck. nat. Ges. in Frankf. a. 

 M. p. LXXIIl. 



Borgert, H. Tiber eine Zweikopfige Kreuzotter. Yerhand- 

 lungen des Xaturwissenschaftlichen Vereins in Ham- 

 burg (3). Band IV. 1896. 



