274 MOLLUSCA.—CLASS IV. PTEROPODA. 
however, our great conchologist was in error ; the Pteropods are not only 
furnished with a distinct mantle, independent of the natatory fins, but the 
parts between which he drew this comparison are not even analogous or- 
gans, the one being destined to perform the office of locomotion, the other 
of calcification. The calcifying property of the mantle is but feebly de- 
veloped in the Pteropoda, the shell, like that of the Carinarie, seldom 
exceeding the common tenuity of fine glass; their power of locomotion 
must, however, be excessive. D’Orbigny, who often passed the night in 
contemplating the myriads of these mollusks that are then seen floating 
on the sea, observes that they begin to appear about twilight, and that 
certain species come to the surface with the utmost regularity at certain 
hours of the night ; he considers that they inhabit particular zones, as it. 
were, at different depths of the ocean, and that they occupy as many 
different periods of time in making their progress to the surface. On 
calm nights, especially in the tropical regions, the surface of the ocean is 
quite darkened with swarms of these prolific animals, and they invariably 
disappear by the return of day-break. The depth of their descent is said 
to be governed in some way by the intensity of light upon the surface of 
the water*; we look upon this theory, however, as postulate rather than 
proof, for their power of vision is described by zootomists as being either 
indistinct or altogether wanting. 
The Pteropoda are separated into two families by De Blainville, Gym- 
nosomata and Thecosomata, naked and conchiferous ; as, however, we 
treat only of the latter, we divide them at once into six genera, as fol- 
lows: 
HyaLz@a. CrESEIS. 
CLEODORA. CuvIiERIA. 
SPIRATELLA. CyMBULIA. 
* Spirits of twilight, whose flight is measured by the opacity of their element. 
