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 coiitortrix, the Cop- 

 perhead, Fig. 1 4, 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 amoimt 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 vertebrae, 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. As the 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 vertebrae, with loss 



