No. 50.] 99 



either too vague for practical use, or not adapted to the American 

 species. We have had, therefore, no alternative but to recast the 

 whole with the extensive material we had on hand. 



The characters upon which the genera adopted are founded, as 

 published in the work containing the results of these investiga- 

 tions*, consist of the numbers, shape, and position of the various 

 plates on the head ; the smoothness or carination of the scales ; 

 the division or unity of the most posterior of the abdominal scu- 

 tellse, and of those beneath the tail, and some other peculiarities. 

 Owing to the difficulty and expense of procuring skeletons of the 

 genera, we found it impossible, at the time, to derive much assis- 

 tance from osteological characters : subsequent examination of 

 some forms, however, has indicated a very decided harmony 

 between the internal and external characters. It is true that some- 

 times slight variations in the number and shape of the plates and 

 rows of scales were detected, even in different sides of the same 

 individual ; but generally the constancy in character was so re- 

 markable as to lead us to rely quite J&rmly on this mode of ar- 

 rangement, and to consider the deviations as abnormal conditions, 

 such as are met with in the most constant types. This view is 

 strengthened by the fact that any variation was rarely symme- 

 trical on both sides ; one side only varying, as a general rule. 

 Of less rank we found to be the number of longitudinal rows of 

 scales on the back and sides. As, too, in nearly every instance, we 

 found the pattern of coloration to be, with very few exceptions, 

 much the same, in the various species of the same genus, as de- 

 termined by the preceding features, we felt warranted in giving 

 the pattern (not the tint) as a secondary, perhaps tertiary charac- 

 ter, of great convenience in grouping species into genera, or their 

 subdivision. 



The number of genera into which the 119 species of N. American 

 serpents described in the catalogue are divided is 35, or an average 

 of nearly 3.4 to 1. The 47 species of Dr. Holbrook are divided 



* Catalogue of North- American Eeptiles in the Museuna of the Smithsonian In- 

 stitution. Part I. Serpents. By S. F. Baied and C. Gieaed. 8vo. pp. 188. Smith- 

 sonian Institution, Washington, January 1853. 



