MUSEUM OF COMPARATIVE ZOOLOGY. 261 



becomes. If a button as it is at birth were to be cut through the con- 

 strictiou 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- 

 tebrae 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, Eaf , 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 vertebrae, still plainly 

 outlined, have consolidated into a single elongate mass, the size of which 

 is being inci*eased 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 vertebrae 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, Linne. 

 Crotahphorus, Linn. ; Caudisona, Laur. 



Figures 5 and 6, from a Crotalus conjluentus, 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 



