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We present the results of ROSAT PSPC observations of 16 Einstein Medium Sensitivity Survey (EMSS; Gioia et al. 1990, ApJS 72, 567; Stocke et al. 1991, ApJS 76, 813) X-ray selected BL Lacertae Objects (XBLs), comprising most of the complete sample presented in Morris et al. (1991, ApJ 380, 49). None of the objects are resolved in their respective PSPC images, but all are easily detected. The spectra exhibited by these objects are quite steep, with energy spectral indices mainly between $\alpha=1.5 -2$, setting them apart from the radio-selected BL Lacs and flat-spectrum radio quasars, which have flatter X-ray spectral slopes. We use these data to recalculate the X-ray luminosity function of these XBLs, as well as their cosmic evolution.
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