790 REPORT OF NATIONAL MUSEUM, 1898. 



Some of the species above admitted are nearly allied, and young 

 specimens are sometimes not readily referred to their proper places. 

 In the first place, although the eyes of young Vertebrata are relatively 

 larger than those of the adult, yet the superciliary plates in this genus 

 encroach more on the frontal in mature than in young specimens, so 

 that in the former the frontal plate is more narrowed posteriorly than 

 in the latter. The color characters of young individuals of Z. lateralis 

 and Z. toiniatus are sometimes not fully developed, so that their refer- 

 ence is difficult. In all of the species the head plates are pale-bordered 

 in the young, and this character may or may not be continued to 

 maturity in Z. tmniatus. Z. constrictor and Z. flagellum are cross- 

 banded and si^otted in youth, but this character disappears except on 

 the anterior dorsal region of the latter species, where it is frequently 

 retained. 



The species are distributed as follows: 



Eastern region : Z. constrictor. 



Austroriparian region : Z. constrictor ; Z. flagellum. 



Central region: Z. constrictor ; Z. tceniatus. 



Pacific region : Z. constrictor; Z. flagellum; Z. ta^niatus; B. lateralis. 



Sonorau region : Z. flagellum; Z. piceus; Z.schottii; Z. lateralis; Z. 

 ornatus; Z. twniatus. 



The number of rows of scales is very constant. Apparent exceptions 

 are referred to under the head of Z. lateralis. The number of labial 

 scuta is very constant except in the Californian representatives of 

 Z. constrictor. The small inferior preocular plate is ver^^ constant in 

 Bascanium, its only absence being noticed in a very few specimens of 

 the Californian form of Z. constrictor. The temporal scales are always 

 normally 2-2-2, and rarely vary from it. 



The anterior and posterior parts of the body are frequently differ- 

 ently colored in this genus. This is especially the case with Z. flagellum, 

 Z. f. semilineatus, and Z. ornatus, where the posterior region is paler 

 than the anterior, and is lacking in the pattern. In Z. constrictor the 

 transition from the black to the green variety is first seen in fading out 

 of the black on the tail and posterior part of the body. 



As regards the striped forms we have evidence how the young differ 

 from the adult in Z. semilineatus and Z. tceniatus. In these the ten- 

 dency to form distinct wider bands is stronger than in the adult, where 

 they are subdivided and more or less obliterated. Thus the young of 

 both these forms resemble more Z. lateralis than do the adults. We 

 may then regard Z. lateralis as representing a primitive form for this 

 series. The primitive form for Z. flagellum and Z. constrictor was 

 probably a cross-banded form, but no such species is>known. In this 

 respect the last-named species resemble the species of the genus Dry- 

 mobius, where the young are cross banded or spotted. Some spotted 

 Drymobii are known where the adults are spotted. 



