894 16. CROTALIDJE 



ed rattle at the tip of the tail. They are provided with large 

 plates along the belly, and the head is covered with large 

 plates or small scales. The eye is well developed, with 

 vertical pupil. These are no rudiments of limbs. Both 

 jaws bear teeth, and near the front of the upper jaw are 

 large, perforate, erectile poison-fangs. 



The two genera which occur in western North America 

 may be distinguished by the following 



SYNOPSIS OF GENERA 



a. Top of head covered by large plates which include a 

 frontal and a pair of parietals. 



Sistrurus. p. 90S. 



a'. Top of head covered by small scales except sometimes on 

 snout; no large frontal or parietals. 



Crotalus. p. 908. 



THE POISON APPARATUS 



"It may not be out of place to refer in this connection 

 to the interesting mechanism of the poison apparatus, as it 

 is a matter not very generally known as yet, though clearly 

 set forth by the researches of specialists, notably Dr. S. Weir 

 Mitchell. The venomous fluid to be injected into a wound 

 made by the teeth has nothing to do with the ordinary 

 saliva, as popularly supposed j nor does the forked tongue 

 or any of the numerous small teeth of the mouth take part 

 in the infliction of the wound. The tongue and smaller 

 teeth are essentially the same as in any harmless serpent. 

 The active instruments are a pair of fangs, one on each side 

 of the upper jaw, rooted in the maxillary bones, which bear 

 no other teeth. The fangs vary in size, being sometimes 

 half an inch long. They are somewhat conical and scythe- 

 shaped, with an extremely fine point; the convexity looks 

 forward, the point downward and backward. The fang is 



