What is the missing character on the seal?
跟進此問題，題中之印


圖略


  Ｘ是何字，解之有難。僅看字形，個人釋之爲「亘」，而@水巷孑蠻釋之爲「亙」，因漢朝既無「亘帝」也無「亙帝」，二解都有問題。


nice try, just some corners of vernacular chinese. practise more, you can master the writing of literary chinese :)

may i suggest:

緣前題雖解．而一疑尚存．固起此問求真

斯印存六字．其五已釋．惟漢後皇前之刻．亘乎．亙乎．眾說紛紜．未審孰是孰非．再者．漢世無亘．亙之諡．益添困惑

back to basic, a seal is for authentication, to make a stamping that other can recognised the stamped document as authentic, from a individual, or an entity.

the seal in question just declare "descendent of a particular emperor of 漢 dynasty".

any mr 劉 can make such claim, so?

next, the text "漢ｘ皇帝後裔" is redundant &amp; ugly. "漢室宗親"; 4 characters are enough for this purpose.

last, 亙 (u+4e99) is suggested, well, because the 漢語多功能字庫 do not has 亘 (u+4e98)

http://humanum.arts.cuhk.edu.hk/Lexis/lexi-mf/search.php?word=亘

and, the 國語辭典 claimed 亘 (u+4e98) as variant of 亙 (u+4e99)

http://dict.revised.moe.edu.tw/cgi-bin/cbdic/gsweb.cgi?o=dcbdic&amp;searchid=W00000070331

i need to read more, before making any sound judgement :(

last, relax, treat it as a "hello kitty stamp" :)

what i want to say is, hello kitty is based on a cat, it does not have a line to represent a mouth, and, it's "tail" is a circle only.