Our viper is the type of a large and poisonous family, but by no means all poisonous snakes are vipers. The deadly cobras belong to a different group, having much more affinity with our own harmless snakes than with the vipers. The rattle-snakes again form a family (_Crotalidæ_) by themselves. There are such things as true sea-serpents, and they are poisonous. They are not, however, allies of any "sea serpent," such as every now and again figures in startling paragraphs in our journals. The true sea-serpents are snakes of small or moderate size, which have their tails flattened from side to side, and which inhabit the Indian Ocean. Of other serpents which are not poisonous, the family of boas and pythons (which kill by crushing) is tolerably familiar to all who have visited zoological collections. There are many beautiful and harmless snakes, such as the families of tree-snakes and whip-snakes, but the snakes which more or less resemble legless lizards are burrowing forms which have the habits and more or less the appearance of earth-worms, such as those which form the families of _Uropeltidæ_ and _Typhlopsidæ_. The last existing reptilian order (_Chelonia_) includes, besides the land tortoises of very various dimensions, a variety of aquatic forms. The best known of these in this country, is the marine family (_Chelonidæ_), to which the edible and tortoise-shell turtles belong. The best known family in the United States and in the Continent of Europe, is the _Emydæ_, to which pertain the terrapins or ordinary river tortoises. Besides these, however, there is a very small family (_Trionicidæ_) of curious and exceptional forms, called mud-tortoises (_Trionyx_). The creatures which have next to be glanced at are those familiar forms, the frogs, toads and efts, which, together with their allies, form another class,--the class _Batrachia_. These animals were long confounded with reptiles but are really widely distinct from them. They are arranged in four orders, three of which have living representatives. The creatures of the first order (the order of tailless Batrachians or _Anoura_)--frogs and toads--exist over almost all the habitable globe; and though the number of their kinds is very great, yet they are all extremely alike in organization. Many kinds (of both frogs and toads) are found to live in trees, the ends of their fingers and toes being dilated to enable them to cling to the surfaces of leaves. The most exceptional species of the whole group are the two tongueless toads, the _Pipa_ of South America and the _Daclytethra_ of Africa, the last-named kind being the lowest of all known animals provided with finger nails.