A few more words descriptive of the external aspect of the _Viperidæ_ may summarize what has already been said of them. Schlegel suggests that their ‘noxious character is expressed in all their parts.’ With the exception of brilliant colouring, this may be accepted as a rule. The broad, flat, angular head, rendering the ‘neck’ thin and conspicuous, has gained for many of them the generic, sometimes specific name of _Trigonocephalus_. From their deadly qualities, _Clotho_, _Severa Atrox_, _Lachesis_, and _Atropos_ are among their names; while _caudalis_ and _brachyura_ describe the short, thin tail as opposed to the long and tapering tails of most colubrines. The true vipers—those that have not the nasal fosse—belong particularly to Africa, the _Crotalidæ_ proper to America, the chief distinction being that the _Crotalidæ_ have and the _Viperidæ_ have _not_ the ‘pit’ (see p. 277), of which more in the next chapter. The rigid, lanceolate scales covering the head are another viperine characteristic; also thick, heavy bodies, tapering at each end, and rough, carinated scales. They inhabit for the most part dry, arid deserts and sandy uncultivated places of the Old World, Africa being their most congenial habitat. The coloured viper and young one convey a good idea of their general aspect. Ophiologists do not agree in the arrangement of genera and species, on account of the forms running so much into each other. Gray gives nine genera and twenty species; Wallace, three genera and twenty-two species; and Dumeril, six genera and seventeen species. The Death adder of Australia (p. 172) is a heterogeneous species. Its aspect is viperine, yet it has not viperine fangs, and does not therefore belong to this chapter. Schlegel thinks it ought not to be separated from the true vipers, but Krefft does not state positively that it is viviparous, so it is altogether anomalous.