What are the rules or instances in which a kana indication of の is dropped?

Question

I've noticed that, in instances such as 「天叢雲剣」{あめのむらくものつるぎ}, の is present in only the pronounced version where it should also be included in the written version. This is not the only instance I have seen of this situation, though I cannot remember the others off the top of my head.

Grammatically, the pronounced version is correct. What exactly are the rules for omitting the の、then?

I assume this is related to the history of the Japanese language; my question is not about why this came to be - just the rules.

Answer 1

I'm not sure if there is a strict rule about this, but AFAIK this happens in some proper nouns of ancient Japanese myths. Probably the most famous example would be ヤマタノオロチ (八岐大蛇). You can find examples in 古事記 and 日本書紀.

Since these proper nouns are really old, no one today know the etymology of these words, and the kanji assigned to these words are basically ateji.

Apparently some people in the past thought it was a good idea to indicate the word boundary by inserting の when reading. This is a convention only found in this limited situation, and we are very unlikely to use this rule to coin a new word.

Another similar usage of "unwritten の" is の found in historical person names. の was pronounced after a clan name (源頼朝 read as みなもとのよりとも). This is discussed in this Wikipedia article. As you can see in the link, the naming convention of historical person names was very complicated, and in general, Japanese students learn these names without wondering when の is required after a "family name".