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 



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. 



