Isthmi is plural. There are actually two plural forms. I will list them from singular to plural.

Isthmus - Singular

Isthmi - Plural

Isthmuses - Plural

The plural of isthmus is accepted as either isthmuses or isthmi.

An isthmus is a narrow region of land. When an isthmus connects two larger landmasses, it can be viewed as a natural path for land transportation or an obstruction for maritime transportation. Many isthmuses (or, if you prefer, isthmi.) are now cut by canals (Suez in the Sinai, Panama, Corinth) or have been considered for cutting (Kra, Tehuantepec...). Some of the USA's largest cities sit upon isthmi: Seattle and Madison, WI.

So, Isthmuses

Good: connect places, regions of commerce.

Bad: Separate oceans/seas, transportation bottlenecks, Hipsters.

According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern I-T-MI. That is, six letter words with 1st letter I and 3rd letter T and 5th letter M and 6th letter I. In alphabetical order, they are:

isthmi

According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern --THMI. That is, six letter words with 3rd letter T and 4th letter H and 5th letter M and 6th letter I. In alphabetical order, they are:

isthmi