B. truncata larvae are nearly round in appearance with a dark band around the margin of the shell. The shell is faintly pink near the umbo and at the ventral margin of large larvae. Two interlocking hinge teeth are present in each valve of larvae 70 μ in length or larger. One tooth is about 25 μ long while the other three are 5–10 μ. Straight hinge larvae measure 55–95 μ in length with a straight hinge line about 45 μ long. Height is 0–10 μ less than length, and thickness 20–25 μ less. The umbo is a rounded knob giving larvae a circular appearance and is present in larvae from 85–315 μ in length. In umbo larvae, height is 0–20 μ less than length, and thickness 25 to 80 μ less. Gills are formed before the loss of the velum, and metamorphosis occurs when larvae reach a length of 250–315 μ.