952 



cast off, are previously deprived of their connection with nutrient 

 fluids by the development of a layer of non-tubular tissue between 

 them and the vascular surface, with which they were originally con- 

 nected and from which they derived their nutriment. 



In order to facilitate description, Mr. Tomes proposes to use 

 several terms as indicative of the arrangement and number of the 

 component tissues of teeth, which he defines and describes at length. 



A minute description of the dental tissues of upwards of fifty 

 species of rodents forms the larger part of the paper. They are 

 taken in the order proposed by Mr. Waterhouse in his arrangement 

 of Rodentia published in Johnston's Physical Atlas. The author 

 has followed this arrangement, because he finds that it accords with 

 the modifications of the dental tissues. The incisors are described 

 as possessing the most strongly marked and constant characters. 



The anterior surface of the incisors of many rodents has a deep 

 brown colour, which has been attributed to the presence of a layer 

 of coloured cementum. The author states that the enamel fibres 

 can be traced through this coloured part to the surface of the tooth, 

 and hence regards the colour as a stain resident in the terminal ends 

 of the fibres, and derives the presence of cementum in this part of 

 the tooth. 



The great distinguishing structural feature in this order of qua- 

 drupeds consists in a lamelliform arrangement of the fibres of the 

 enamel in the inner division of that tissue. 



The author then enters into an elaborate and detailed account of 

 the peculiarities of this sti-ucture, as exhibited in the SciuridcB^ in 

 the diff^erent members of the family Muridce in the Hystricine family, 

 the Leporidce, &c., and finally considers the following conclusions 

 as established by these details : — 



" That the teeth of some species of the order have specific struc- 

 tural characters by which they can be distinguished from any other 

 known teeth. That in the teeth of all the Rodentia, excepting the 

 family Leporidae, a portion of the enamel has a lameUiform arrange- 

 ment of its fibres ; that the enamel lamellae have a different and 

 distinctive character in each of the larger groups, and that the variety 

 of structure is constant throughout the members of the same group ;" 

 — " and that the variety in the structure of the dental tissues, with 

 a few isolated exceptions, justifies and accords with the arrangement 

 of the members of the order into the several divisions proposed by 

 Mr. Waterhouse, and deduced by him from the relations of the 

 several parts of the skull." 



It is stated at the conclusion, that the author purposes in a future 

 communication entering into the development and the special adap- 

 tation of the peculiarities of the dental tissues which it has been the 

 business of this paper to lay before the Society. 



2. " On the Meteorology of the Lake District of Cumberland and 

 Westmoreland, with a continuation of the results of experiments 

 on the fall of Rain at various heights, up to 3166 feet above the 

 Sea-Level." By J. F. Miller, Esq., F.R.A.S. Communicated by 

 Lieut.-CoL Sabine, R.A., For. Sec. R.S. 



