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POTAMIDIDAE 151
the outside of the body whorl only in the region behind the slightly flaring
Hp. 3 other cords are present just below the suture. Columella area dished;
reinforced by the round, pillar-like columella. A small pimple is present near
the posterior canal in the aperture. Outer lip with strong crenulations on the
inside. Periostracum a soft gray-black felt. A common littoral species in
the West Indies which bears its young in a brood pouch. Rare in Florida.
Figure 38. Modulus viodiihis Umnt (southeast United States and the West Indies).
a, side view of living animal; b, ventral view showing foot, head and under edge of
mantle; c, operculum; d, apertural view of shell. X2. (From Abbott 1944 in
Modulus modulus Linne
Johnsonia.)
Family MODULIDAE
Genus Modulus Gray 1842
Florida to Texas and the West Indies.
Atlantic Modulus
Plate 2 if; figure 38
About /4 inch in length. Characterized by the small, projecting, tooth-
like, frequently brownish spine located on the lower end of the columella.
Base of shell with about 5 strong, spiral cords. Top of whorls with low,
slanting, axial ribs. Color grayish white with beach-worn specimens often
exhibiting flecks of purple-brown. Found abundantly among weeds in shal-
low, warm waters.
Modulus carchedonius Lamarck, the Angled Modulus (Caribbean area)
lives in deeper water, and differs in having the periphery of the shell well-
angulated, the spiral cords smaller and neater, in lacking the strong, axial ribs,
and in never having the columella tooth colored. Not too common.
Family POTAMIDIDAE
Subfamily POTAMIDINAE
(Horn Shells)
Genus Cerithidea Swainson 1840