and so bold an assertion are well considered, not hastily advanced. I have before me the left anterior molar, taken from the jaw of a colt which was rising four. A representation of the tooth alluded to is here given. [Illustration] _a_ denotes the crusta petrosa, _b_ the outer surface of the enamel, the lines representing the plated or finely grooved aspect, which this part exhibits on its exterior. _c_, which letter will, on inspection, be discovered in the centre of the darkened space to the right of the lower portion of the enamel, indicates a spot where disease was existing, and to which reference will be hereafter made. _d_ denotes the roots, which, as will be seen, are (although the tooth had been in active employment for twelve months) still incomplete, and not enveloped by crusta petrosa. The above wood-cut, the accuracy of which is not to be disputed, shows a comparatively large portion of the assumed secretive membrane to be absent. The deficient part of the outer covering, I may here state, was removed by a natural process, as I myself extracted the tooth. Nevertheless where the crusta petrosa is deficient, the laminated external surface of the enamel is exposed in perfect and normal condition. This evidence appears pretty conclusive, for where the secretive substance is absent, the secretion would hardly appear. On the other hand, if the secretion of the enamel were the special function of the crusta petrosa, then wherever the one was found, the other would also be present. Let the reader, however, refer to the wood-cut inserted at page 21. In that diagram, the crusta petrosa is faithfully represented as thickest at the root; but at this very point the enamel is seen to be entirely wanting. The conclusion towards which such facts point, is almost decisive; but, nevertheless, there are many inferences which help to support it. If a longitudinal section of a tooth be made, a very thin white line may be plainly seen, indicating, probably between the ivory and the enamel, another source for the production of the substance which the crusta petrosa has been said to secrete. Moreover, if a tooth be boiled for some hours, a very little force will enable any person to pull off the enamel from the ivory; and this experiment likewise suggests the intervention of some gelatinous membrane between the two structures. The microscope also confirms the opinion I advance, and shows that, in the old tooth, the membrane is absorbed, for its remains can only be detected in isolated places. I am fully aware that none of the arguments here advanced are, with the exception of the first, at all conclusive. Appearances are deceptive, and results consequent upon artificial processes by no means to be trusted. After all, however, the necessity for such a secretive membrane is by no means imperative for the production of the enamel. Our views on these points may be too circumscribed, since at the extremities of the