848 



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 Cape?isis, 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 ursimis may be distinguished from 

 the Z>. 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 — 



Macropus giganteus. Petaurus sciureus. 



Hypsiprymnus penicillatus. Dasyurus macrourus. 



minor. ursinus. 



Phalangista vulpina. Thylacinus cynocephalus. 



Phascolomys Wombat. Didelphis virginiana. 

 Petaurus taguanoides. 



The author considers that the facts stated in his paper justify two 

 conclusions of a general character : first, that the existence of pro- 

 longed and fully-developed tubes in the enamel, continuous with 

 those of the subjacent dentine, is comm.on to the great majority, if 

 not all, of the marsupial animals, excepting the Wombat ; and, se- 

 condly, that the enamel and dentine are so closely related, that they 

 should be regarded as modifications of each other, rather than as 

 tissues of a wholly different nature. 



10. " On the Motion of Gases."— Part II. By Thomas Graham, 

 F.R.S. &c. 



The experiments described by the author in the former paper on the 



