Missing Constellations in VM Zodiac

[tavie]

Here is a reference to the 10-month calendar ( see under 19, page 371 ):

Plutarch, Live of Numa

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Macrobius is also said to have discussed the calendar in the Saturnalia. However, I cannot find a specific passage (so far).

Book 1, Chapter 12.  But Macrobius would have been using earlier sources, as likely was Plutarch.  The earliest I can see is in a work by Varro (116-27 BCE), which is a bit ambiguous but does seem to imply that there were 10 months originally, and then 2 added on.  Livy also mentions Numa's calendar reform but I don't think he refers to Romulus's 10 month system.  I think the best is Ovid, who literally tries to provide excuses for Romulus's choice (including that 10 months was the gestation period for a child...), and Ovid was probably easier for a writer in the early 1400s to get hold of than Plutarch.  At least assuming the writer was in Europe.

If the 10 month system is a folk explanation of something confusing with the calendar, I would think the origin of the confusion dates a lot further back from 153 BCE. 

In any case, I'm struggling to see how it would have been seen as authoritative, including because it was old.  I think generally the 12 month reform by "Numa" is presented as progress, more efficient, with the implication that Romulus's system was not.  

It also seems out of step with medieval astrology.  Perilous days were still calculated for Jan and Feb.  If the VM had an agricultural focus, then a 10 month calendar could be explained by the VM author wanting to incorporate only relevant months for agriculture.  But that seems to me to be more likely stemming from their own choice, rather than being inspired by Romulus via Ovid.