COX S BAZAR (Reuters) - Grieving Rohingya Muslim refugees buried 18 of their own, including children, on Friday, hours after they drowned off the Bangladesh coast where their boat capsized in heavy seas.   Relatives wept after they identified the bodies in a makeshift morgue and took them away to be buried in a mass grave.    Our boat hit a big rock and it turned over,  said Lalu Miya, whose wife, daughter and two sons died in the accident late Thursday. Another two children are missing. Suna Miya said he lost three daughters, aged 10, 8 and 3, in the accident. His wife and three other children survived. As men prepared the burial ground in daylight, relatives squatted around babies covered in cloth. Hands reached out to touch the children. Miya bent over his daughter to cradle her head, and kissed her goodbye.   More than 60 refugees from Myanmar are believed to have drowned, the latest victims in what the United Nations says is the world s fastest-developing refugee emergency.   A surge in the numbers of Rohingya fleeing the Myanmar military campaign has taken the total to more than half a million in Bangladesh. Abul Kalam, 55, who survived, said his wife, two daughters and a grandson were dead. Kalam said armed Buddhists had taken away his livestock and food a week ago. He said villagers had been summoned to a military office and told that there were no such people as Rohingya in Myanmar.   The refugee boat capsized as darkness fell, in driving wind and rain, and high seas.   An official with the International Organization for Migration said 23 people were confirmed dead and 40 were missing. Seventeen survived. 