The researches of Dr. Weir Mitchel of Philadelphia have been of great value to ophiologists. For two whole years he gave the best portion of his time to the study of rattlesnakes, having a number of them under constant observation. An exhaustive paper by him was published in the _Smithsonian Contributions_, Washington, D.C., in 1860, giving details of experiments with the venom and the treatments adopted. But of especial interest here are his observations on the fangs and their volitional action, it having previously been supposed that the mere opening of the mouth brought the fangs into position, which is not the case. As the _Crotalus_ can move each side of its mouth independently, so it can use one or both fangs. ‘When the mouth is opened widely, it still has perfect control over the fang, raising or depressing it at will.’ Dr. Mitchel saw that though both fangs were present, both were not always used. When a viperine snake yawns extensively, as it so often does, you may sometimes perceive the fangs partially erected or entirely so, or the ‘vibratile motion’ in them observed by Fayrer. When the snake is angry, this vibratile action is much like that of a cat gnashing the teeth; but when only in a yawn, the partial and unequal erection of one or both fangs has the appearance of being involuntary. In this I speak from observation. The effect is similar to that seen about a person’s mouth in trying to suppress a yawn—a sort of convulsive, nervous twitching. Whatever the cause, you perceive the fangs moving, but _not_ moving always in accord. The shedding or replacement of the fangs is, Dr. Mitchel thinks, a regular process, as in the teeth of some fishes, though not regular as to time. Sometimes, but not always, they are shed with the casting of the cuticle. He ‘cannot suppose that the almost mature secondaries are awaiting an accident;’ which agrees precisely with the opinions of Dr. Edward Nicholson and other physiologists quoted in the last chapter: ‘A crop of young teeth’ (or of fangs) ‘work their way into the intervals of the old teeth, and gradually expel these latter.’ When lost by accident or by violence, therefore, the process of replacement is slower, as we can readily conceive, the ‘secondary’ next in turn not being as yet ready for duty. Though the American scientific journals devoted to zoology are rich in ophidian literature, there are few available to English students; and I regret I am unable to ascertain from across the Atlantic the latest researches and conclusions regarding this and several other correlative points. To Professor Martin Duncan I am indebted for the loan of a volume which forms one of the ‘Bulletins’ of the United States Geological Surveys, containing a valuable ‘Report’ on the Crotalus by Dr. Elliot Coues, of the United States army, late surgeon and naturalist to the United States Northern Boundary Commission, 1878.