8 BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



continued interest in its progress, and for many helpful suggestions 

 and criticisms, particularly upon the general method of approach. 



I am also greatly indebted to Dr. Leonhard Stejneger for the privi- 

 lege of studying the material in the United States National Museum; 

 to Mr. A. E. Brown for the opportunity of examining the specimens 

 in the Academy of Natural Sciences of Philadelphia, and to Messrs. 

 Whitmer Stone and James A. G. Rehn for assistance in doing po. 



I am particularly indebted to the Field Museum of Natural His- 

 tory in Chicago, through Dr. S. E. Meek, for the loan of their excel- 

 lent series of specimens, which has furnished the solution for several 

 knotty problems that otherwise must have remained obscure. For 

 the loan of the material in the American Museum of Natural History 

 I am indebted to the Director, Dr. H. C. Bumpus. 



My thanks are also due to Dr. Raymond Pearl, Agricultural Experi- 

 ment Station, Orono, Maine, who suggested the method employed 

 in plotting the variations in scutellation in the different forms. Most 

 students of systematic zoology avoid statistical methods, which is 

 unfortunate, as a graphic description is always much clearer than a 

 written one. Simple diagrams, similar to the ones used in this paper, 

 are easy to prepare, and summarize in the clearest and most concise 

 manner the geographic variations in a form, and should be em- 

 ployed whenever possible. It may be well to say here that in these 

 diagrams X denotes the arithmetical mean, the heavy lines the range 

 of variations, O the mid-point of the range (the average of the two 

 extreme individuals) , and the numerals in the diagram the number 

 of specimens; also that the stars on the maps do not represent the 

 exact locality from which specimens have been examined. 



I am also under obligations to many persons for aid in securing 

 living specimens from localities from which records were desirable. 



GENUS THAMNOPHIS. 

 DESCRIPTION. 



Scutellation. — The genus Tliarrmoplns of Fitzinger (1S43, 26), 

 established upon the Coluber saurita of Linnaeus (1766, 385), and 

 later defined as Eutaenia by Baird and Girard (1853, 24), belongs to 

 the family Natricidae, and differs from its nearest American relative, 

 Matrix, by the absence of scale pits and the presence of an undivided 

 anal plate. 



The cephalic plates are normal, consisting of paired internasals, 

 prefrontals, occipitals, and parietals, and a single frontal. A loreal 

 plate is present; the rostral normal in form and the nasals divided, 

 the nostril being between them. There is usually a single preocular 

 on either side, but in three forms (elegans, ordinoides, and hammondi) 



