CAPT OF AN ACR LGT FAILS TO INSURE THAT HIS FMS STARTS THE DSCNT FOR AN ALT XING RESTR.

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WE HAD BEEN CLRED FROM FL370 TO FL290 AND THE AUTOPLT/FMS WAS ENGAGED AND MAKING THE DSCNT. UPON LEVELOFF THE FLT ATTENDANTS CAME INTO THE COCKPIT TO OFFER BEVERAGES. DURING THE LEVELOFF THE FMS MODE CHANGED FROM VNAV TO ALT HOLD. AS EVIDENCED BY LATER EVENTS; I DID NOT NOTICE THE FMS MODE CHANGE. SHORTLY AFTER LEVELING AT FL290 WE WERE CLRED TO CROSS OTT VOR AT FL250. I PROGRAMMED THE FMS AND SET THE ALT ALERT WINDOW FOR THE XING RESTR. LATER; WHEN I SHOULD'VE BEEN STARTING THE DSCNT I WAS ENJOYING THE VIEW OUT THE WINDOW AND LOST SITUATIONAL AWARENESS. I DID NOT START THE DSCNT. VNAV; WHICH I NORMALLY ONLY RELY UPON AS A BACKUP FOR DSCNTS COULD NOT START US DOWN BECAUSE THE FMS HAD PREVIOUSLY CHANGED TO ALT HOLD. AS A RESULT OF THE DISTRACTIONS AND LOSS OF SITUATIONAL AWARENESS I DID NOT START THE DSCNT UNTIL ALMOST OVER OTT VOR. THE FO ASKED IF WE WERE SUPPOSED TO CROSS AT FL250; WHEREUPON I BEGAN AN IMMEDIATE DSCNT. WE CROSSED OTT AT LEAST SEVERAL THOUSAND FT HIGH. APPARENTLY THERE WAS NO CONFLICTING TFC IN THE AREA (AT LEAST AS FAR AS WE COULD TELL FROM TCASII) AND THE CTLR SAID NOTHING ABOUT THE XING ALT. IT HAS BEEN SEVERAL YRS; AT LEAST; SINCE I'VE HAD AN ALT 'BUST.' FORTUNATELY; I WILL BE ABLE TO LEARN FROM THIS EVENT AND PAY BETTER ATTN IN THE FUTURE.