華甲[kakou] aged 61
介甲[kaikou] crust-like shell
機甲[kikou] armour (e.g. tank)
龜甲[kikou] tortoise shell

甲乙[kouotsu] A and B
甲乙丙[kouotsuhei] ABC
甲殻[koukaku] carapace
甲殻類[koukakurui] crustacean

月と鼈[tsukitosuppon] like chalk and cheese (superficially similar, but completely different) (the moon and a turtle shell are both round, but they are otherwise completely different)
鼈甲[bekkou] tortoiseshell (of the hawksbill turtle)
鼈甲色[bekkouiro] amber (colour, color)
石鼈[hizaragai] chiton (any marine mollusk of the class Polyplacophora)
鼈甲蜂[bekkoubachi] spider wasp

田鼈[tagame] Lethocerus deyrollei (species of giant water bug)
鼈茸[suppontake] common stinkhorn (Phallus impudicus)
鼈甲細工[bekkouzaiku] article of tortoise shell
鼈甲飴[bekkouame] hard candy