I'm not 100% sure but I think that those are un formed seeds as I tend to find a lot in citrus I get, grapefruits, seville oranges and blood oranges. I imagine that most citrus seeds are at least quater of a dime in size but don't quote me on that. Also Ugli fruit don't come true to type as they are mono ebryonic but more information on them, and tangelos, can be found at
www.hort-purdue.edu . Maybe there is better sites out there but this is all I know so please correct me if I am wrong,
Sincerely,
James