Your page at http://www.cut-the-knot.org/blue/Abacus.shtml uses the misconcieved plural for abacus i.e abaci. I quote a paragraph from http://moonflare.com/abacus/index.html#1.3 .
"The plural of abacus has a been a matter of heated debate for some time. Webster's Dictionary, the equivocal American reference, gives abaci as the plural. This was done in analogy to words descending from Latin such as cactus and fungus, since in Latin there is a noun form with singular ending -us and plural ending -i. This has been one of the few Latin pluralisations to persist where ones like formulae have fallen into disuse.

But on the other hand, the equivocal British reference, the Oxford Dictionary, gives the plural of abacus to be abacuses. As odd as this sounds, it is more in tune to the actual roots of the word, which are not at all Latin, but Arabic. Although the Arabic pluralisation is not used, I believe it's reasonable to say that it doesn't make sense to pluralise a non-Latin word as though it were Latin, masking its etymological roots. For this reason, I choose this plural in this document."

To further cement this interpretation I also quote from the online Oxford Dictionary for 'abacus' at http://www.askoxford.com/concise_oed/abacus?view=uk which states,
"abacus
/abbkss/

• noun (pl. abacuses) a frame with rows of wires or grooves along which beads are slid, used for calculating.

— ORIGIN Greek abax ‘slab, drawing board’."

Hope this helps to resolve this minor slip.

>Hope this helps to resolve this minor slip.

It can't be a slip if it is supported by a widely used dictionary. It is a matter of discussion or, perhaps, of a controversy, but by no means a slip.

>>Hope this helps to resolve this minor slip.
>
>It can't be a slip if it is supported by a widely used
>dictionary. It is a matter of discussion or, perhaps, of a
>controversy, but by no means a slip.

And http://www.merriam-webster.com/dictionary/abacus says either is OK, but lists abaci first.

It also states: Etymology: Latin, from Greek abak-, abax, literally, slab.

So some authorities believe there is a Latin link.