CIRRHIPEDA. 



39 



15. Acasta. — Shell oval, rather conical, 

 composed of pieces which can be separated. 

 Cone formed of six unequal 

 valves united together, the base 

 an orbicular valve, concave on <^gj| 

 the inside, resembling a small 

 cup. Operculum formed of four pieces. — 4 re- 

 cent species. 



These animals are found lodging in sponge, 

 and are never attached to hard bodies. They 

 inhabit tropical seas. 



16. Ceeusa. — Shell orbicular, having four 

 valves, which are unequal and 

 united together. Operculum 

 internal, and consisting of four 

 valves. Animal rather globu- 

 lar, and having three or four 

 pair of arms. — 4 recent species. 



These small shells are found affixed to ma- 

 drepores, and other substances in the Northern 

 and Indian Seas. 



1 7. Pyrgojsta. — Shell 

 rather globular, ventricose, 

 convex above, apex perfo- 

 rated, aperture small, and 



D 4 



