CHAP. IV. IN ANIMALS. 93 the tittering of monkeys should be a rapidly reiterated sound, cannot be explained. During the utterance of these sounds, the mouth is transversely elongated by the corners being drawn backwards and upwards; and of this fact an explanation will be attempted in a future chapter. But the whole subject of the differences of the sounds produced under different states of the mind is so obscure, that I have succeeded in throwing hardly any light on it; and the remarks which I have made, have but little signifi- cance. All the sounds hitherto noticed de- pend on the respiratory organs; but sounds produced by wholly different means are likewise expressive. Rab- bits stamp loudly on the ground as a signal to their comrades; and if a man knows how to do so properly, he may on a quiet evening hear the rabbits answering him all around. These ani- mals, as well as some others, also stamp on the ground when made angry. Por- cupines rattle their quills and vibrate their tails when angered; and one be- haved in this manner when a live snake was placed in its compartment. The quills on the tail are very different from those on the body: they are short, hollow, thin like a goose-quill, with their ends transversely truncated, so that they are open; they are supported on long, thin, elastic foot- stalks. Wow, when the tail is rapidly shaken, these hollow quills strike against each other and produce, as I heard in the presence of Mr. Bartlett, a peculiar Fio. 11. — Sound - pro- ducing1 quillfl from tlj.o tail of the Porcupine.