Two basic shapes of the hinge line are recognized including a straight or sway-backed form (shell is decidedly D-shaped), and one that is rounded with a protruding bump or umbo in the center. Knowing the shape of the hinge line is the first step in identifying one of the four basic types of shelled larvae or veligers. These include the straight-hinge form, umbonal form, pediveliger, and plantigrade. These basic types of shelled larvae are separated based on hinge development, shell shape, shell size, and the presence or absence of a foot and velum. Knowing the shelled larval type can greatly aid in the identification of the veligers.
