"-trix" is also a suffix in the English language denoting a female agent of a given action. It is the counterpart to the male/neuter suffix "-or." Thus, a female director would be a directrix, a female inheritor an inheretrix, and a female dominator a dominatrix. The last example is probably the most common usage of this suffix, although I actually did hear a British narrator on History Channel International refer to Queen Elizabeth as an inheritrix earlier today. Otherwise, "-trix" is somewhat rare in common usage, although for no good reason that I can see.
This suffix is related to "-ess" in meaning and sound (e.g. seamstress), but the two are not interchangeable. "-trix" is only felicitously used when it can be interchanged with its male counterpart "-or." Given the somewhat arbitrary nature of the English language, this usually has to be decided on a word by word basis. | 
