| The spines in both rows increase in size away from the umbo and there are a few rare scattered additional spines on the ventral corpus. |
| It is by the position of the umbo in the grave that this fact has been exactly ascertained. |
| 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. |
| Larvae larger than 210 µm have a more protuberant umbo, and their sides form obtuse angles with the shoulders. |
| 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. |
| When the larvae reach this size, the umbo looks like a round protuberance hiding the straight hinge line. |
