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We present updated and complete radio, optical and X-ray data for BL Lacs inthe Einstein Medium Sensitivity Survey (EMSS). The complete "M91" sample firstpresented in Morris et al. is updated to include 26 BL Lacs in total and wedefine a new, virtually complete sample consisting of 41 EMSS BL Lacs (the"D40" sample). New high signal-to-noise, arcsecond-resolution VLA observationsare also presented for eleven EMSS BL Lacs, completing VLA observations of theM91 sample. The addition of four new objects, as well as updated X-ray flux andredshift information, has increased the v/v_max value for the M91 sample to0.399\pm0.057; and v/v_max = 0.427\pm0.045 for the newly defined D40 sample. Inconjunction with other studies of X-ray-selected BL Lac (XBL) samples, theseresults solidify negative evolution for XBLs, especially for more extremehigh-energy-peaked BL Lacs. The observed v/v_max, spectral and radio propertiesof XBLs are completely consistent with being the beamed population oflow-luminosity, FR-1 radio galaxies. However, our VLA observations do confirmthat XBLs are too core-dominated to be consistent with a beamed population ofFR-1s seen at intermediate angles, as suggested by the unified model, if XBLshave moderate outflow velocities (gamma approx 5).Comment: 32 pages, 63 figures, accepted A

The Properties of the X-Ray-selected EMSS Sample of BL Lacertae Objects