The skeleton is composed of two segments. Internal skeletal structure is composed of MB, A, D, L, V, DL,V L and AL. In some specimens, right or left (or both) l, Al, Dl, Ll and one VL-VL are present. The first segment is bell-shaped to hemispherical and distinguished from the second segment by DL and VL and the furrows along them in most specimens. When 1, Al, Dl and VL-VL exist, the boundary dividing the two segments consists of VL, Ll and Dl. At the dorsal side of the first segment, the shell tends to be double-layered. The inner layer includes the apical rib, and the outer one is a cobweb-like meshwork.

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