| In general, the septalium is a trough-shaped structure in the umbonal region of the dorsal valve. |
| Dorsal valve only slightly sulcate, with very shallow sulcus originating in the umbonal area. |
| Ventral valve lateral profile anterior to the umbo gently convex with maximum height at the umbonal region. |
| Its shell is considerably twisted just below the umbonal region, while its whole surface is densely striated. |
| Ventral sulcus and dorsal median fold moderately developed, originating in umbonal area. |
| Results revealed that after four months larvae that had only reached the umbonal stage by Day 15 were significantly smaller than those that had reached the pediveliger stage. |
