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Macrothrix mexicanus sp . nov. is described from central Mexico, a transition zone between the nearctic and neotropics. All localities where it was found are over 1800 meters above sea level . It shows many resemblances with M. laticornis, M. camjatae and M. rosea but is characterized by a persistent dorsal tooth on the valve keel, a spinous papilla on the basipodite of the antenna, the second thoracic limb with a long conical sensillum between scraper 1 and the gnathobase, the endopod of trunk limb IV having two setae ; the postabdomen with the dorsal margin bilobed, and the distal segment of the seta natatoria which is unusually long .

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Objectives: To compare the distribution of chemical elements among the carious and normal enamel layers on teenagers, using energy dispersive X ray spectrometer (EDS Rx). Materials and Methods: The EDS Rx analyzer is integrated to scanner electron microscope. The macro and microelements were made in 30 premolars of teenager: 14 carious enamel layers and 16 normal enamel layers. Results: The quantitative and qualitative microanalyses of macro and microelements were found between the enamel layers of carious and normal premolar in term of variation and concentration expressed in percentage atomic weight. The statistical data analysis of ANOVA showed that the macroelements [C, Ca, P] and the microelements [Al, Cl, Mg, Na] were significantly different (P < 0.05) while, the macro [O] and the microelements [In, Si, W, S] were not significantly different (P < 0.05) among the carious and normal enamel layers. Moreover, the microelements Sb,