MUSEUM OF COMPARATIVE ZOOLOGY. 267 
rattlers, is obtained through that of the end bone, the thick skin cover- 
ing the latter, and the mode of growth. Generally, on the pointed tip 
there is no chance to retain the cap during and after the slough. With- 
out the backward growth from the front border at the time of forming 
the cap, again, the rattle would not exist, as each new cap would be 
formed entirely within its predecessor. Ancistrodon contortrix, the Cop- 
perhead, Fig. 14, gives a hint of the probable manner of origin of the 
rattle, in the folds, or swellings, at the front border of the cap. The 
presence of these folds apparently indicates that growth from the mar- 
gin has taken place. If, as seems to be the case from the folds, the 
necessary manner of growing already exists, but a slight increase in 
it, increasing the amount of the swellings and constrictions, would be 
needed to provide the Copperhead with a rattle. After the retention of 
the displaced cap was secured, the change from the habit of striking 
the tip upon the ground to simple shaking would be followed by loss of 
flexibility in the tail itself, by rigidity of the column, —a condition, 
with inclusion in the cap and the peculiar strain on the muscles, favor- 
ing consolidation of the terminal vertebre, and tending to draw the 
spines forward. It appears very much as if the rattle originated in 
some such way. 
Though the Copperhead has been specially used as an illustration 
here, it is not asserted that the rattlesnakes are directly derived from it. 
Taking the general characteristics into consideration, it seems more 
likely they took origin in several stocks; one of them, allied rather 
closely to Ancistrodon, yielding the Sistruri (the small rattlesnakes 
with large crown shields) ; another, nearer to Lachesis, giving rise to 
Crotalus durissus and allies. 
In summarizing, we may say the rattlesnakes have probably been de- 
rived from members of the same family that had no rattle. The button 
of the rattle was formed by enlargement of the terminal cap covering 
the terminal bone, very likely without fusion with scales. The shape 
of the button was determined by that of the bone and skin of the tip; 
it is modified in the second and following rings by the ring immedi- 
ately preceding. The exterior or exposed part of each button after the 
first, is formed in front of the ring with which it is in contact, and 
pushes the latter back. Asthe button is displaced to become a ring, 
it is prevented from passing entirely off by the swollen skin, com- 
pletely filling its anterior chambers, behind the constrictions. The 
development of the button and the rattle was accompanied by a consoli- 
dation and compacting of a larger number of the vertebre, with loss 
