I've installed Mongo 3.2.0 on OS X and get the following error calling:

mongod --storageEngine inMemory --dbpath <my_path>

exception in initAndListen: 18656 Cannot start server with an unknown storage engine: inMemory, terminating

Long version
2016-01-04T20:12:24.344+0100 I CONTROL [initandlisten] db version v3.2.0
2016-01-04T20:12:24.344+0100 I CONTROL [initandlisten] git version: 45d947729a0315accb6d4f15a6b06be6d9c19fe7
2016-01-04T20:12:24.344+0100 I CONTROL [initandlisten] allocator: system
2016-01-04T20:12:24.344+0100 I CONTROL [initandlisten] modules: none
2016-01-04T20:12:24.344+0100 I CONTROL [initandlisten] build environment:
2016-01-04T20:12:24.345+0100 I CONTROL [initandlisten] distarch: x86_64
2016-01-04T20:12:24.345+0100 I CONTROL [initandlisten] target_arch: x86_64
2016-01-04T20:12:24.345+0100 I CONTROL [initandlisten] options: { storage:

}
2016-01-04T20:12:24.345+0100 I STORAGE [initandlisten] exception in initAndListen: 18656 Cannot start server with an unknown storage engine: inMemory, terminating
2016-01-04T20:12:24.345+0100 I CONTROL [initandlisten] dbexit: rc: 100