On the motion of gases.—Part II

mammalian teeth, subject only to one of two exceptions; In which exceptions, however, the teeth are small and may readily be distin guished from marsupial by their external character. They are the teeth of the Hyrax Capensi , the British Shrews, and the molar teeth of the Jerboa. The author states, that so far as he has had opportunities of ex amination, the teeth of the various species may also be distinguished, the one from the other. He points out, for instance, that, on com parison, the teeth of Dasyurus ursinus may be distinguished from the D. macrourus. The peculiar characteristic of marsupial teeth exists in the con tinuation of the dentinal tubes into the enamel; so far as the author has investigated them, he finds but one exception, and that in the Wombat,—the representative of the rodents in the marsupial order. This creature, he finds, has teeth that are nearly allied in structure as well as external form to the teeth of rodents, and more especially to the Hare and Rabbit. The author states, that he has observed that the dentinal tubes in the human and other teeth are sometimes continued for a short distance into the enamel. This he considers a rudimentary condi tion which is fully developed in the marsupial teeth. The author observes that the dentinal and enamel pulp become firmly united to each other previous to the commencement of calcification in either, and that it is highly probable that the linear columns of the two pulps are joined end to end, and that the columns of the enamel pulp so joined become developed into tubes instead of into solid enamel fibres. He considers this the more probable, as he has observed that the enamel fibres in an early stage of development are partially tubular in the teeth of several animals whose enamel fibres are ulti mately solid. The teeth described and figured are those of the—