Garman.] 
173 
[Jan. 16, 
dred in some of the excessively elongate types. On the copper- 
heads and moccasins ( Ancistrodon ) the number may be estimated 
at from forty-five to fifty -five, and on the rattlesnakes ( Crotalus 
and Sistrurus) from twenty-five to thirty-five. 
The epiderm of the rattlesnake is shed in much the same way as 
that of other snakes. It comes off the whole body, and is all thrown 
away excepting what covers the end of the tail. The cap is set free 
from the skin in a measure, but it cannot get entirely away because 
of its shape. Though loosened and dislodged from its place of 
growth, by its form it is mechanically retained to become a part of 
a rattle. The tip of the tail in one of these serpents is called a “but- 
ton each of the sloughs (caps) from the button forms a “ring” of 
the rattle. With modification and thickening of the skin in the but- 
ton the epiderm has become thicker and firmer than over the balance 
of the body ; the structure and mode of growth are apparently the 
same. The character of the organ is such that it is necessarily a post- 
natal acquisition, yet its existence at the later period is assured 
by the condition of the button at birth. In fact, specialization has 
already proceeded so far when the creature is born that the inquiry 
must be taken up in much earlier embryonic stages. 
In an embrj’O showing the body and tail well formed, previous to 
the appearance of the scale folds on the skin, the vertebral column 
in the tail is similar to that of other ophidians at the same stage : 
each vertebra is distinct, movable, and supplied with the usual mus- 
cles. The folds are seen first anteriorly, on the body, and later they 
appear on the tail ; so that by the time the cap is formed on the but- 
ton, the scales are completed over the whole skin. This stage is 
illustrated in figs. 1 and 2. Externally they show a short, broad, 
rounded cap, without shoulders or constrictions, of a shape inter- 
mediate between that of the pointed-tailed species and that of young 
rattlesnakes just before the first slough, and distinct^ and regular- 
ly separated from the folds in front of it, or, in other words, with- 
out sign of having been enlarged b} T fusion of scales. The condition 
within the button is shown in fig. 2. The vertebral column is flex- 
ible ; the vertebrae are distinct and surrounded by muscles. At 
this time the embryo appears to have attained the condition in which 
the majority of the serpents are hatched ; but it still makes a con- 
siderable advance before birth. It is as if there had been a later 
addition to the embryonic development, resulting in important 
changes of structure. An advance is made beyond the point reached 
