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292 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 Figure 64c Arctic Seas to Cape Cod, Massachusetts. 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 Figure 646 Cape Cod, Massachusetts, to Brazil. (N. Lat. 42° to S. Lat. 28°). 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.