MUSEUM OF COMPARATIVE ZOOLOGY. 261 
becomes. Ifa button as it is at birth were to be cut through the con- 
striction immediately behind the anterior swelling, the hinder portion 
would correspond with the cap as seen in Figures 1 and 2. There is no 
evidence of any fusion of scales with the button around its front border, 
Except in case of the button, there is externally very little difference 
between this stage and the next represented. Within the cap the ver- 
tebree are distinct, slightly smaller than those just in front of them, 
like the latter surrounded by muscles, and the skin is thicker than else- 
where. Between this stage and the following the anterior portion of 
the cap appears to be added by backward growth at the front margin, 
like that which la‘er in life displaces the older button to make way for 
the new. 
Figures 3 and 4 are drawn from young ones of the same species, 
S. catenatus, Raf., eight and a quarter inches in length, about a week after 
birth. In them the button has been perfected, the cap having gained, 
as compared with Fig. 2, all the portion anterior to the constriction, 
On several of these specimens there is a tendency to fusion and irregu- 
larity among the scales immediately in front of the button, but in no 
case is there any disposition on the part of the scales to fuse with the 
latter. A portion of the button corresponding to the externally visible 
part of each ring has been acquired, while the entire length has in- 
creased a couple of inches, in a short time just before birth. Inside of 
the button the changes have been greater: the vertebra, still plainly 
outlined, have consolidated into a single elongate mass, the size of which 
is being increased by both lateral and terminal growth; the vertical 
processes have grown together ; and the muscles have been displaced by 
the enlarging bone and the thickening skin. Muscular command of the 
individual vertebre within the button has been lost in the consolidation, 
but the muscles of the tail retain a firm hold on the mass, and the loss 
finds compensation in a better means of agitating the rattle. For later 
stages we are compelled to turn to a closely allied genus. 
Crotalus, Linné. 
Crotalophorus, Linn. ; Caudisona, Laur. 
Figures 5 and 6, from a Crotalus confluentus, Say, fourteen inches in 
length, show a considerable advance from the preceding. The specimen 
was taken, with the third button about half grown, when the process of 
pushing back the second ring was well under way. The first ring had 
