No. 562] VABIATIONS IN NUMBER OF VERTEBRJE 627 



form of a thesis to the faculty of the department of litera- 

 ture, science and the arts, University of Michigan, in 

 partial fulfillment of the requirements for the degree of 

 Master of Arts. The work was completed by the junior 

 writer in 1912. The skiagraphs were made by Dr. E. T. 

 Loeffler, of the department of dentistry, University of 

 Michigan, and we gratefully acknowledge his untiring 

 efforts to provide us with satisfactory plates of the rather 

 difficult material, without which the work could not have 

 been completed. 



The methods used in this study are simple. Two 

 closely related species belonging to the North American 

 genus Regina (Natrix in part or Tropidonotus in part of 

 some writers), R. leberis (L.) and R. grahami (B. & G.) 

 were used. Most of the work was done on R. leberis. In 

 a series of specimens the sex and the number of ventral 



(belly and subcaudal) scales was determined, and skia- 

 graphs were made of each specimen from which the 

 vertebrae were counted. Before making the skiagraphs a 

 pin was thrust through the body at the anus so that the 

 position of the last few body vertebrae would be revealed, 

 and it was determined by dissection which pair of the 

 short anterior ribs is the first to reach a ventral scute and 

 the number of this scute. 



General Relations of Vertebra and Scutes— It is first 

 to be stated that in the few specimens dissected it is the 



