LIST OF ILLUSTRATIONS. 



Page. 

 Thamnophis ordinoides elegans (Tule Lake, Oregon). (Pliotograph by Finley 



and Bohlman) Facing 1 



Fig. 1. Diagram illustrating the arrangement of the dorsal scale rows in the 



garter-snakes 18 



2. The arrangement of the head plates in the genus Thamnophis, as repre- 



sented in an individual possessing the maximum number of labial 

 plates for the genus 22 



3. Diagram showing the method of variation in the number of labial scutes 



that results in the formulas characteristic of the different forms of 

 garter-snakes 23 



4. Thamnophis megalops (1098 Field Museum), showing the normal 



arrangement of the labial scutes when the formula is ^^ij 25 



5. Thamnophis megalops (1098 Field Museum), showing the small size of 



the fourth supralabial when tliere are nine in this series 26 



6. Thamnophis ordinoides (1109 Field Museum), showing the reduced 



third supralabial — the first stage in the reduction from 8 to 7 26 



7. Thamnophis sauritus (32972 University of Michigan Museum), showing 



the normal arrangement of the labial scutes when the formula is | 26 



8. Thamnophis ordinoides (1109 Field Museum), showing third supra- 



labial reduced and partly fused with the fourth to leave seven 27 



9. Thamnophis sirtalis (30820 University of Michigan Museum), showing 



the normal arrangement of the labial scutes when the formula is y% 27 



10. Thamnophis butleri (226 Ruthven Collection), showing the large fifth 



supralabial caused by the decrease in size of the penultimate scute 



and its fusion with the antipenultimate labial 27 



11. Thamnophis radix (155 Ruthven Collection), showing sixth supralabial 



narrower than usual and ending on the first instead of the second 

 temporal 28 



12. Thamnophis butleri. showing the last stage in the loss of the sixth supra- 



labial — the fusion of the reduced scute with the fifth . 28 



13. Distribution of Thamnophis megalops, as indicated by the locality 



recoi'ds 48 



14. Diagram showing the variation in the dorsal scale formula in Tham- 



nophis megalops 49 



15. Diagram showing the variation in the number of supralabials in Tham- 



nophis megalops 50 



16. Diagram showing the variation in the number of infralabials in Tham- 



nophis megalops 50 



17. Diagram showing the variation in the number of ventral scutes in 



Thamnophis megalops 51 



18. Diagram showing the variation in the number of subcaudal .scutes in 



Thamnophis megalops 52 



19. Diagram showing the variation in the proportionate tail-length in 



Thamnophis megalops 53 



20. Santa Cruz River at Tucson, Arizona. Thamnophis marcianus has 



been found here. (From Ruthven's Reptiles and Amphibians from 

 New Mexico and Arizona) 59 



IX 



