st as a thin layer coating the phragmocone).The hollow region at the front of the guard is termed the "alveolus" 护甲后端的中穴内包覆或连接着圆锥状的，而闭锥就是箭石调节浮力的气室，大多数箭石标本的护甲末端中穴与闭锥部份常会被压碎，没有完整的保存下来，只有保存较好的标本能保留下来。Projecting forwards from one side of the phragmocone is the