A curious species (the boa canina) has a large head, shaped somewhat like that of a dog; the general colour a bright Saxon-green, with transverse white bars down the back. The sides are of a deeper green, and the belly is white. Wallace describes a small one only eleven feet in length, but as thick as a man's thigh. It was secured by having a stick tightly tied round the neck. It went about dragging its clog with it, sometimes opening its mouth with a very suspicious yawn, and sometimes turning its tail up into the air. Being put into a cage, and released from the stick, it began to breathe most violently, the expirations sounding like high-pressure steam escaping from a Great Western locomotive. The boa, however, is not much dreaded in South America, as it seldom or never attacks man; which the anaconda is said always to do, if it can find him unprepared. Stories are told of desperate encounters between travellers in the forests of the Amazon and pythons or boas. A French traveller narrates how, on one occasion, the whole of his attendants took to flight on seeing a huge python approaching,--with the exception of a gallant native, who, attacking the monster vigorously with a long, lithe pole, struck it a blow which paralysed its powers; when, the party returning, it was easily killed. THE RATTLESNAKE. Venomous as is the bite of the rattlesnake, and abounding as it does in all parts of the continent, it is less dreaded than many other serpents. It is, in the first place, very sluggish in its habits; and it is happily compelled to bear about it an instrument which gives notice of its approach and intention of biting. The South American rattlesnake-- the Boaquira crotalus horridus--has the rattle placed at the end of the tail. It consists of several dry, hard, bony processes, so shaped that the tip of each upper bone runs within two of the bones below it. By this means they have not only a movable coherence, but also make a multiplied sound, each bone hitting against the others at the same time. The rattle is placed with the broad end perpendicular to the body, the first joint being fastened to the last vertebra of the tail by means of a thick muscle under it, as well as by the membranes which unite it to the skin. Indeed, an idea of this curious structure may be formed by placing a number of thimbles one within the other. These bony rings increase in number with the age of the animal; and they are generally found with from five to fourteen. The sound produced has been compared to that of knife-grinding. It cannot be heard at a distance, and in rainy weather is almost inaudible. The effects of the bite vary according to the season of the year; indeed, at times it will seldom strike a foe, and the venom is comparatively mild in its effects. At other times the poison is of deadly intensity, and, should a large vein be bitten, the victim speedily dies.