Fam. 35.] NATURAL HISTORY SOCIETY. llo 



In- 10 or 12 cross ridges which give the edges a serrated outline ; 



stalk about equnl to the blade in length, and J the width. Shell 



L. 6 ; H. 6. Pallet L. 12 ; W 2 ; Stolk^7.— Mass. and south. Rare. 



CLASS 11. GASTROPODA. 



Head distinct, usually furnished with eyes and tentacles ; 

 body, in most species, protected b}^ a spiral or conical univalve 

 shell, rarely naked, never with a bivalve shell ; lower surface of 

 animal developing a thickened, expanded, creeping disk or foot, 

 except in the aberrant order Pteropoda. 



Order PTEROPODA. 



Small, free swimming, brightly colored, gregarious, pelagic 

 mollusks usually naked, but sometimes protected by a thin ex- 

 ternal or internal often membranaceous shell. The peculiarity 

 which gives them their name is the wing-like projections used 

 in swimming ; the head is small ; the mouth small, sometimes 

 tentaculate, and furnished with a radula. The sexes are united 

 in the sam.e individual. Very .abundant in the Arctic seas ; 

 certain species furnish the chief "food of the whale. They 

 come to the surface mainly in the evening, and occasionally 

 have been found quite abundant near shore. The scope of this 

 book hardly needs their introduction, but a few species are given. 

 Suborder THECOSOMATA. 



Animal furnished with a shell; head rudimentary. 

 ^ Family 34. LIMACINID/E. 



Represented mainly by the genus : — 

 LiMACiNA, Cuv. 



Shell present, external, minute (length 1-5), spiral, sinistral, 

 transparent; operculum paucispiral, glassy. Animal with nar- 

 row, simple fins attached to the sides of the mouth and united 

 ventrally ; mouth central, with prominei;it lips. Pelagic. 



Limacina retroversa, Fleming. (Fig. 21). Shell ovate globose, 

 very thin, pellucid ; umbilicus deep, narrow ; whorls 7 with mi- 

 nute revolving lines ; aperture equals the spire. L. 2J ; D. If. — 

 Arctic seas to N. J. Several other species are known but the 

 shells are rarely ever seen on shore. 



Family 35. CAVOLINIID/E. 



Shell straight or sligbtly curved, globular or needle-shaped ; 

 no operculum ; animal with two large fins. 



