| Larvae larger than 210 µm have a more protuberant umbo, and their sides form obtuse angles with the shoulders. |
| They are sometimes bell-shaped and then flattened, often with a mound or umbo. |
| Argonauta attaches the egg-mass to the umbo of the shell near the dorsal arms, not in the mantle cavity or posterior parts of the shell. |
| The oldest part of the shell, the umbo, can be recognized as a large hump on the anterior end of the dorsal side of each shell. |
| The cuticle is smoky olive to fuliginous, darker when young, becoming paler as the pileus expands, but always darker on the umbo. |
| Later, when the larva reaches 115 µm, the umbo moves to the side, becoming offcentred and protuberant. |
