Convective Accessory clouds and weather - eg Wall Clouds, Roll clouds, Lightning, Hail, Mammatus etc
This link can help to a certain extent with your cloud identification.
This type of cloud feature forms under most convective clouds but is most dramatic under large CB clouds. The cloud consists of bulges which is where the mamma comes from (latin for udder), and is formed when a downdraft carries cold air down into the warmer layer causing it to cool rapidly and form water droplets, causing an upside down type of convection. You can see this type of cloud under any 'us' (i.e Cirrocumulus, the 'us' part is put on all types that are convective) cloud but in smaller clouds is usually distorted or not that visually amazing, but can look very spectaular under the back edge or side of a thunderstorm anvil. They can occur at any levels from very low to very high in the atmosphere, the anvil picture below demonstrates how it can form very high up in the atmosphere on the leading edge of a developing anvil.
Here are some of my picture(s) of this cloud: