894 16. CROTALIDA 
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. 905. 
a.—Top of head covered by small scales except sometimes on 
snout; no large frontal or parietals. 
Crotalus.—p. 908. 
Tue Porson Apparatus 
“Tt 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; 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 
