We may reasonably conclude that the _Crotalus_, in common with other snakes, also with dogs and cats, expresses a variety of feelings with its sounding tail, fear being the most predominant one. The Indians recognise its utility as a warning by gratefully abstaining from killing one that rattles. They superstitiously regard it as protective to themselves if not to the snake, and they in turn carefully protect the reptile. Backwoodsmen display little or no fear when they _hear_ the _Crotalus_, and though they do not spare it, regard it with less bitter animosity than they display towards its cousin the Copper-head; because, as a facetious writer has testified of it, ‘it never bites without provocation, living up to the laws of honour, and by his rattles giving challenge in an honourable way.’ That the sound has a language of its own is known by the fact that when disturbed and one rattle is sprung, all other rattlesnakes within hearing take up the chorus. That the sexes also understand each other through crotaline eloquence is generally believed. In fact, to each other and to themselves they have, no doubt, as many variations in the use of their rattles, as any other animal in the expression of its tail; and probably all the above enumerated examples are at one time or another its legitimate uses. Those who have most closely observed them have detected a variety of cadences in one and the same rattle. Those also who have carefully watched rattlesnakes under various circumstances, must perceive that timidity is one of the strongest features in this reptile. In chap. xxx. I will give examples of this. Already convinced by observation, I attributed to excessive timidity the chief agitation of the rattle, when writing on the Ophidia in the _Dublin University Magazine_, December 1875, and again in _Aunt Judy’s Magazine_, July 1877. Fear causes some snakes to puff themselves; others to expand or flatten the body; fear excites the cobra to erect its anterior ribs and display its ‘hood;’ and, above all, fear causes most snakes to hiss. Fear is coupled with anger, in these attempts to do their best towards repelling the offender. Dr. E. Coues, in speaking of the rattle, supposes it to have possibly ‘resulted in the course of time from the continual agitation of the caudal extremity of these _highly nervous and irritable creatures_.’ Dr. Weir Mitchell has known captive snakes to vibrate the rattle for hours at a time; and probably, if there were opportunities of becoming more intimately acquainted with crotaline idiosyncrasies, we should discover some snakes to be more or less afflicted with temper, nervousness, terror, or other emotions which induce an animal to express its feelings in its own way.