Lexington (Mass.) Contributory Retirement System is searching for a manager to run $10 million in active domestic opportunistic equities, comprising mainly midcap core, said Eric W. Stewart, consultant with Meketa Investment Group, which is assisting. Officials of the $92.8 million fund will consider proposals for either a commingled fund or a separate account. They are adding the mandate for diversification. Funding will come from reducing existing equity mandates; no managers will be terminated. Plan officials will likely schedule final presentations for the second quarter. Marguerite Oliva, administrator, referred questions to Meketa.