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.
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2016-10-10T04:17:18.357
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2016-10-10T12:24:51.250
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question
grammar nuances readings
What are the rules or instances in which a kana indication of の is dropped?
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](https://en.wikipedia.org/wiki/Yamata_no_Orochi)
(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](https://japanese.stackexchange.com/q/13788/5010).

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](https://en.wikipedia.org/wiki/Japanese_name#Historical_names). 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".
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answer