Often the handiest way to track down a mystery genus is in the index of an extensive publication such as Vaught, the Treatise on Invertebrate Paleontology, Goto and Poppe, etc.  If your collection emphasizes New Zealand, Beu's catalog of Cenozoic mollusks may be quite helpful.

In several cases, the name can be suggestive.  Later genera often resemble earlier names.  For example, Ittybittium is closely related to Bittium.  On the other hand, either wide use of a particular root or changing ideas about relationships can throw this off.  Mya, Planktomya, Myonera, Panomya, Cuneamya, Solemya, and Camya are bivalves, but not close relatives.

As Tom pointed out, the double i comes from the original genus name plus the idae, inae, etc. ending.  Usually, the final a, us, or um gets dropped from the genus to form the root.  Thus, Lyonsia is in Lyonsiidae, Trochus in Trochidae.  There are a few grammatical peculiarites, such as Glycymeris in Glycymerididae.  Also, occasionally a problem comes up.  Trapezium should grammatically be in the Trapeziidae.  However, Trapeziidae is also the proper family name for an arthropod.  I believe that the bivalves are officially Trapezidae, but cannot find a reference immediately.  On the other hand, the molluscan Harpidae (from Harpa) was kept and the trilobite Harpidae, from Harpes, was changed.

    Dr. David Campbell
    "Old Seashells"
    Biology Department
    Saint Mary's College of Maryland
    18952 E. Fisher Road
    St. Mary's City, MD  20686-3001 USA
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"Mollusks murmured 'Morning!'.  And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!