[thaumasiotes]

The prefix in "decimal" is "decim", like you'd expect given that the root is "decem". There is no "dec-" prefix.

(You do see "deca" used as a prefix, "a" included, but that doesn't come from Latin.)

[amenghra]

You are right. I should have written: (septem)ber was the 7th month of the year, (octo)ber was the 8th, (novem)ber the ninth, and (decem)ber the tenth!