PRAIRIE RATTLESNAKE. 673 
explained by supposing that the venom accumulates owing to the fangs 
being shed. 
Various theories have been advanced as to the use of the rattle, 
none of them being entirely satisfactory. One supposition is that it has 
resulted in time from the agitation of the highly nervous tail, and has 
no special function. Pityophis melanoleucus, the Bull Snake shows such 
an agitation, and its tail when in motion is said to somewhat resemble 
the rattle of this species. Again, it has been conjectured to be of use in 
calling the sexes together, and thus aiding in the preservation of the 
species. Another hypothesis is that, as it resembles the noise made by 
a grasshopper, it might be of service in decoying insectivorous birds 
within reach of the animal. The rattle is said to be heard often when 
there is no cause of irritation. If such be the case, it probably does not 
occur with the Crotalide in our limits. A fourth suggestion is that it 
serves to alarm the higher species, giving them warning, and thus en- 
abling them to escape its bite. Such a hypothesis is contrary to all 
analogy, since nature’s mode is to provide the animal with that which 
is best for its individual wants without regard to others. A last supposi- 
tion is that it is of use to the serpent in terrifying its enemies. Proba- 
bly this last is more worthy of credence, and is not to be set aside by 
the fact that it sometimes attracts the attention of enemies in such a 
way as to induce them to destroy it. In other cases it renders the 
snake more frightful, and enables it to paralyze its foes or desired prey 
with fear. It may also be of some use in the other ways mentioned, 
especially in bringing about the union of the sexes. 
A popular belief is that one rattle is added each year; however, this 
cannot be true, as Dr. Holbrook knew a case of two rattles being added 
in a year, and Dr. Bachman observed a species in which four were 
developed in twelve months. Thus, we may see that the number of rate 
tles is, as Dr. Holbrook observed, dependent upon the condition of the 
animal as regards liberty, nourishment, etc., and is no test of age. 
The oil of this species is said to be very valuable. 
Genus CROTALOPHORUS. Linnazus. 
Cephalic region above covered with plates, which are seen posterior to a transverse line 
between the eyes ; caudal rattle always present, though smaller than in Crotalus ; body 
moderately strong; pupil vertical; rostral plate moderately large, temporals and labials 
small ; the two anterior inframaxillaries large, the remainder small. 
Dorsal rows of scales 26. : C. TERGEMINUS. 
Dorsal rows of scales 23. : ; : : : i ‘ C. MILIARIUS, 
43 
