from Okinawa.com "Turtleback Tombs or Mother's Womb Tomb
There are two types of tombs you will see while on Okinawa: the turtleback tomb (kameko-baka) and the gable type. The turtleback tomb is easy to spot due to its distinctive shave (resembling the back of a turtle's shell). The tombs line the hills and streets and can be seen in a variety of sizes nearly everywhere on the island.
Kameko-baka serve as ancestral vaults for holding the remains of the deceased in carefully arranged urns. The rectangular opening located in the center-front of the structure is a conspicuous feature you will find in all tombs and serves as an entrance for placing urns within. These tombs are also built in the shape of a woman's womb. Buddhists believe the reasoning behind this is to place the deceased back where they originally came from: the womb. You can still see the turtleback tomb, but it is slowly being replaced by the cheaper, space-saving, condominium-style tombs.
Each April (around March 15th on the lunar calendar), Okinawans participate in memorial services for ancestors in an event known locally as Shiimii (known as Seimei Sai in Japanese). Blood relatives gather in front of tombs in a sort of family reunion where they open special containers of Okinawan ceremonial foods and drink beer and awamori together. This event is a joyful outing that strengthens family ties."
Itami, Japan, 1951 & Kyoto, Japan