THE VARIATIONS EXHIBITED BY THMINOPHIS ORDI- 

 NOIDES (BAIRD AND GIRARD), A GARTER-SNAKE IN- 

 HABITING THE SAUSALITO PENINSULA, CALIFORNIA. 



By Joseph C. Thompson, 



Surgeon, United States Navy. 



INTRODUCTION. 



The garter-snakes of the Pacific coast found west of the Sierra- 

 Nevada-Cascade Range, from Vancouver in the north to the Teha- 

 chapi Mountains in the south, have been assembled in the Memoir of 

 Dr. A. G. Ruthven, under the one name of Thamnophis ordinoides 

 (Baird and Girard).^ 



Tliis species presents a remarkably large series of variations, is 

 equaled by no other in the genus, and is only approached by T. ele- 

 gans and T. radix which occupy regions five to eight times greater 

 in extent. 



The specimens upon which tliis study is based were captured on 

 the Sausafito Peninsula, which forms the northern boundary of the 

 Golden Gate, the entrance to San Francisco Bay. They were all 

 taken within a radius of 3 kilometers. 



METHODS. 



In addition to enumerating the number of scale rows on the various 

 parts of the body, it has been found that most instructive records 

 may be obtained if note is taken of the exact gastrostege level at 

 which an added row begins or a suppressed row ends. 



Assuming that a normal specimen is being examined, the following 

 is about what may be expected: At the beginning of the neck there 

 may be counted 10 rows of scales on each side of the median, a total 

 of 21 rows; a little further back there are 9 rows and the median, a 

 total of 19 rows; where this reduced count begins it will be seen that 

 it is caused by the IV row (counting the row next to the gastrostege as 

 the first row) being suppressed, and this occurs usually at the level 

 of the sixth gastrostege. From tliis point on there are 19 rows until 

 about the twenty-fifth gastrostege, where the count is increased to 21 

 rows; this is brought about by the addition of a row on each side, 

 the added row being the V. This V row, with the accompanying 

 total of 21 rows, continues to about the sixty-fifth gastrostege, when 

 the V row is suppressed and the count of 19 rows is resumed. The 19 

 rows continue to the level of the eighty-fifth gastrostege, when the 



1 Bull. 61, U. S. NationalMnseum, 1908, p. 147. 



Proceedings U. S. National Museum, Vol. 47-No. 2051. 



351 



