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I to 1% inches in length. Primary ribs i6 to i8, fairly distinct in the young stages; sculptureless in the senile stage. Round in cross-section. Chalky-white when eroded. Common from 20 to 1,000 fathoms. Dentalium autillarum Orbigny Antillean Tusk South half of Florida and the West Indies. About I inch in length, roundish in cross-section. Primary ribs 9, but increasing to 1 2 near the middle and finally with 24 near the aperture. Mi- croscopic, transverse lines between the ribs. Color opaque-white, rarely reflecting a greenish tint. Encircled with weak, zigzag bands or splotches of translucent gray. Dentalium pilsbryi Rehder Pilsbry's Tusk Figure 69d West Florida and Brazil only. % to 1% inches in length, roundish in cross-section. Primary ribs 9, with a smaller, weaker, rounded, secondary rib appearing between each. All ribs fade out toward the anterior end. Intercostal spaces flat, crossed by coarse growth lines. No transverse, microscopic sculpture. Color opaque- white; without gray splotches. Formerly known as D. pseiidohexagonum Henderson 1920, not Arnold 1903. Uncommon from 2 to 5 fathoms. Dentalium pretiosum Sowerby Indian Money Tusk Figure 6gi Alaska to Lower California. About 2 inches in length, moderately curved and solid; opaque-white, ivory-like, commonly with faint dirty-buff rings of growth. Apex with a short notch on the convex side. A common offshore species which was used extensively by the northwest Indians for money. Subgenus Fissidentalium Fischer 1895 Dentalium floridense Henderson Florida Tusk Southeast Florida and the West Indies. 2 to 3 inches in length, roundish in cross-section. Shell hard and yel- lowish white. Apex hexagonal with concave spaces between. Ribs increase to 24 anteriorly and are rounded, equal-sized and crowded. There is a long, narrow apical slit on the convex side. Rare from 35 to 100 fathoms.