1901.] NATURAL SCIENCES OF PIIILA DELI'HIA. lOt 



Xotwitlistanding the wide range of this genus, through the 

 ■whole of Auiorica from lower Canada to Brazil, its members form 

 a very compact group and though many of the species resemble 

 each other closely, there is a curious absence of transitional charac- 

 ters, so that it is necessary to recognize as distinct species, forms as 

 closely similar as adamanteus and atrox, as well as confluentus and 

 oregonus (= lacifer B. and G. ), in which the differences, though 

 slight, are, as far as I can discover, absolutely constant. 



Key to the Species oj the United States. 



A. — Anterior nasal in contact with rostral: 



(I. — Back with chevron-shaped cross-bauds; tail black. 



I. C. horridus. 

 b. — Back with spots; or cross-bands posteriorly: 



a\ — Rostral as high or higher than wide; 3-5 scales be- 

 tween suboculars and labials : 

 a'. — Dorsal spots lozenge shaped: 



Lozenges distinct; a light vertical line in front 

 of nostril; bands on tail not very distinct, 



2. C. adamanteus. 



Lozenges with angles cut off; no light line in 



front of nostril; tail white with black bands, 



3. C. atrox. 



l\ Dorsal spots rhomboid ; cross-bands behind : 



Head scales larger; dark streak beginning at 



anterior corner of eye, . 4. C confluentn^. 



Head scales smaller; dark streak beginning at 



posterior corner of eye, . 5. C. oregonvs. 



(.,_ Dorsal spots with a light centre on each side of 



the median line, .... 6. C. molossm. 



^l\ Dorsal spots small, in two rows, . 7. G. pricel. 



ji_ Rostral wider than high; 2 scales between sub- 

 oculars and labials : 



(t^ Supraoculars not produced into a horn: 



Spots anteriorly; cro?s-bands behind, 



8. C. tigris. 

 Greenish, with black cross-bands, 



9. C. lepidus. 



j\ Supraocular produced into a horn, 10. C- cerastes. 



B. Anterior nasal separated from rostral by scales, 



II. C mitchellL 



Crotalus molossus B. and G. 



I. e., 10 ; Cope, I. c, .689, and Rep. Nat. Mus., 1154 ; Stej., I. c, 424 ; 

 C. lerrificus (part) Boxil., I. c, III, 573. 

 Snout broad; rostral rather small, its width about equal to its 

 height; scales on top of the muzzle larger than in any other North 



