THE VARIATION EXHIBITED BY THAMNOPfflS ORDI- 

 NOIDES (BAIRD AND GIRARD), A GARTER SNAKE 

 INHABITING THE SAN FRANCISCO PENINSULA. 



By Joseph C. Thompson, 



Surgeon, United States Navy. 



INTRODUCTION. 



The more one reviews the literature of the garter snakes of North 

 America, the more one becomes impressed with the necessity of ascer- 

 taining the complete range of variation that may be exliibited by each 

 of the well-established species in the genus. To determine this it will 

 be requisite to collect a fair series of specimens from a restricted local- 

 ity and to record its slightest variations. Similar studies will have to 

 be made of sets from widely separated regions. In order to facilitate 

 the comparison of the data an endeavor should be made to conform 

 to a uniform method of tabulating the figures and presenting the 

 facts. 



In the past T. ordinoides has been divided into as many as eight 

 species and subspecies; this alone imphes that the form is subject to 

 considerable variation. What this variation really amounts to can 

 only be appreciated by one who has seen large series from all parts of 

 its range, for it is difficult to believe that the dwarfed and sombre- 

 hued examples from the north, with the minimum scale count of 

 17-15, are one and the same species as the large brUUantly colored 

 specimens from the south or the veritable giant from the Santa Clara 

 VaUey, whose body alone measures 1,040 mm. and has a maximum 

 scale formula of 21-23-21-19-17. 



Dr. Ruthven in his Memoir on the Garter Snakes of North Amer- 

 ica^ recognizes TTiamnopliis elegans (Baird and Girard) 1853, and 

 Thamnopliis ordinoides (Baird and Girard) 1852, as distinct species. 

 In speaking of T. elegans ^ he wiites : " . . . its western Hmit 

 can not be drawn exactly owing to the fact that it intergrades with 

 another form ..." referring to T. ordinoides. In speaking of 



1 1908, Bull. 61, U. S. National Museum. 2 idem, p. 143. 



Proceedings U. S. National Museum. Vol. 52-No. 2179. 



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