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292 A?nerican Seashells
and possibly present in the Lower Florida Keys. This species is a sand-
dweller.
Order PTEROPODA
(Sea Butterflies or Pteropods)
These small, pelagic gastropods are very abundant in the open seas in
nearly every part of the world. They are occasionally washed ashore, but
more commonly their shells are found in dredge hauls. The identification of
pteropods is important to many types of oceanographic studies. There are
two suborders, Thecosomata or those having shells, and the Gymnosomata
or those without shells. We have omitted the latter group, and refer inter-
ested workers to our bibliography. Every known American species (eastern
Pacific and western Atlantic) of the shelled Thecosomata has been included
and figured.
Suborder THECOSOMATA
Family SPIRATELLIDAE
Genus Spiratella Blainville 1817
{Limacina)
Spiratella helicina Phipps Helicid Pteropod
Figure 64a
Arctic Seas to the Gulf of Maine. Arctic Seas to northern California.
Up to 8 mm. in length, spire short, shell wider than long. Surface with
relatively large, axial threads. Adults (over 3 mm.) without an operculum.
Abundant enough in the Arctic Seas to serve as an important source of food
for certain whales. S. pacifica Dall is the same.
Spiratella retroversa Fleming Retrovert Pteropod
Arctic Seas to Cape Cod, Massachusetts.
Figure 64c
Up to 5 or 6 mm. in length, spire slightly elevated, umbilicus distinct,
shell higher than wide. Entire surface covered with fine, spiral hues. 10
whorls. Limacina balea Moller and Spirialis goiildi Stimpson are this species.
Spiratella trochi^omtis Orbigny Trochiform Pteropod
Cape Cod, Massachusetts, to Brazil. (N. Lat. 42° to S. Lat. 28°).
Figure 646
I mm, in length, very close in characters, except shape, to 5. retroversa,
and thought by some workers to be a warm-water subspecies of that species.