THE VARIATION EXHIBITED BY MAINLAND AND ISLAND 

 SPECIMENS OF THE HIBAKARI SNAKE, NATRIX VIBA- 

 KARI (BOIE). 



By J. C. Thompson, 



Surgeon, United States Navy. 



In a recent paper * attention was called to the fact, first made 

 known by Wallace, that where a serpent inhabits both the mainland 

 and adjoining islands individuals captured on the islands frequently 

 possess the larger number of vertebrae. This has been examined into 

 and found to hold true for a great many species, including members 

 of the Colubridas, Najidse, and Crotalidae. An endeavor is being 

 made to ascertain exactly which portion of the vertebral column 

 becomes involved in the process of lengthening. In the present 

 species it is the caudal region alone. 



The island specimens of Natrix mbakari (Boie) have been reported 

 from Hondo, Shikoku, and Eaushu, three of the four principal islands 

 of Japan. None have been reported from Hokkaido, the northern 

 island. The mainland specimens have been captured at Khabarovka, 

 Vladivostok, and Possiet Bay in the Ussuri Province of Siberia, and 

 from Fasan in southeast Korea. 



If the sum of the gastrosteges and the urosteges in each specimen 

 be plotted in linear fashion, it may be seen at a glance that the 

 examples from the islands have the larger number of vertebrae. 



Diagram showing the variation in the sum of the ventrals and subcaudals. 



i i • • • 



In the above diagram the circles represent mainland and the dots 

 island specimens. Wliere the sex has been recorded, there is added 

 the conventional sign. 



1 Herpetological Notices, No. 3, San Francisco. 



Proceedings U' S. National Museum, Vol. 4ft— No. 2020. 



157 



