192 
SNAKES. 
there are several shields on the same region of the head, while the number of rows 
of shields beneath the tail is only from twenty to forty-one. The best known re¬ 
presentative of the genus is the common boa, or boa-constrictor ( B. constrictor), 
which ranges in South America from Venezuela to upper Argentina. At times 
reaching as much as 12 feet in length, it has the muzzle slightly prominent in the 
adult, although obliquely truncated in the immature state. In general colour it is 
pale brown on the upper-parts, with from fifteen to twenty dark brown crossbars, 
which expand inferiorly, sometimes to such an extent as to become connected on 
COMMON boa nat. size). 
the sides of the body, and thus to surround oval or elliptical spots of the light ground¬ 
colour ; the expanded portion of each bar having a light longitudinal line. On the 
sides are a series of large light-centred dark brown spots, most of which alternate 
with the crossbars; and on the tail all the markings become relatively larger, of a 
brick-red colour, margined with black, and separated by yellowish intervals. From 
the muzzle to the nape runs a dark brown median streak, widening posteriorly, where 
it may be looped; another bar of the same colour passes on each side of the head 
through the eye, while there is a third below the latter, and the lips are marked 
by such bars; the rostral shield of the snout being also ornamented with a crescentic 
blackish mark. The under-parts are yellowish, with spots and dots, or merely dots, 
of black. The whole tone of coloration is dull, sombre, and adapted to harmonise 
with the shades of brown, black, and yellow on the bark of tropical forest trees. 
