It seems almost a pity to kill such nice little animals, for as a Gaucho said, while sharpening his knife on the back of one, "Son tan mansos" (they are so quiet).
Of reptiles there are many kinds: one snake (a Trigonocephalus, or Cophias, subsequently called by M.Bibron T.crepitans), from the size of the poison channel in its fangs, must be very deadly.
Cuvier, in opposition to some other naturalists, makes this a sub-genus of the rattlesnake, and intermediate between it and the viper.

In confirmation of this opinion, I observed a fact, which appears to me very curious and instructive, as showing how every character, even though it may be in some degree independent of structure, has a tendency to vary by slow degrees.

The extremity of the tail of this snake is terminated by a point, which is very slightly enlarged; and as the animal glides along, it constantly vibrates the last inch; and this part striking against the dry grass and brushwood, produces a rattling noise, which can be distinctly heard at the distance of six feet.

As often as the animal was irritated or surprised, its tail was shaken; and the vibrations were extremely rapid.

Even as long as the body retained its irritability, a tendency to this habitual movement was evident.
This Trigonocephalus has, therefore, in some respects the structure of a viper, with the habits of a rattlesnake: the noise, however, being produced by a simpler device.