Skull are not showing at the mobs

rua:
  Mobtype: squid
  Display: 'Dinnerbone'
  Health: 1
  Disguise:
   Type: witherskeleton
  Skills:
   - potion{t=INVISIBILITY;d=2147483647;l=1} @self ~onSpawn
   - equipskull{skull=turtle_head} @self ~onSpawn 1
  Options:
    PreventRandomEquipment: true
    PreventOtherDrops: true
    Silent: true
    MovementSpeed: 0.1

turtle_head:
  Id: 397
  Data: 3
  Options:...