EXPLANATION OF THE PLATES. 



As the general report on the reptiles of western North America, observed by the different 

 exploring parties, has been excluded from the series for want of room, all that can be given 

 here is an explanation of the plates prepared for this report. These represent the details of 

 external form in different species of North American serpents. All the outlines from the same 

 specimen have a common number, to which is added a system of lettering common to all, 

 and indicating the several details^ as follows : 



M, the upper surface of the head. 



I, the lower surface of the head. 



s, the side of the head, (usually the left.) 



/, the head from before. 



a, the anal region, showing the peculiar scutellation. 



sc, the scales on the side of the body, showing several oblique rows, with the upper portion 

 of the abdominal scales. Each figure illustrates usually the median dorsal row and 

 those between it and the abdominal plates on the left side. The entire number of 

 dorsal rows of scales will therefore be twice the number represented, less one. The 

 figure is generally taken at about the posterior end of the anterior third of the length 

 of body, including head and tail. 



In some cases a different series of letters has been adopted ; these are explained in their 

 place. 



The figures have, as far as possible, been taken from the type specimens of the species, 

 especially those described in the catalogue of serpents in the museum of the Smithsonian 

 Institution, (1853,) to which the page column refers. 



