These are plurals that do not follow the general rule of simply adding an ‘s’ for example: cars, houses, lawyers etc.
Lets look at a few, some have more than one possibility:
Index: indexes, indices
There is more than one index, remember to check all the indexes/indice
Memorandum: memoranda, memorandums
We received numerous memoranda/memorandums from our client.
Notary public: notaries public
There are many notaries public available to do this.
Prospectus: prospectuses
Please send out some prospectuses to these clients regarding our legal services.
How about you, do you know of any other irregular plurals?