272 MOLLUSCA.—CLASS IV. PTEROPODA. 
Crass IV. PTEROPODA. 
Animal pelagium, subgelatinosum, parte corporis abdominali testa in- 
clusum ; capite indistincto, tentaculis duobus instructo, at inter- 
dum nullo; pallio amplo, tenuissimo, ad utrumque latus affixo ; 
ore subterminali, alis membranaceis, natatoreis, duabus, utrinque 
munito. Branchie pectinate, per aquam tantim respirantes, intro 
corpore celate. 
Testa parva, aut vitrea, aut gelatinoso-cartilaginea, vel globosa, 
vel trigona, vel cylindrica, partem corporis abdominalem obtegens. 
By referring to our observations on the preceding class of conchiferous 
mollusks, the Mollusca Gasteropoda, it will be seen how much indebted 
we have been to the great author of the ‘ Regne Animal’ for his very 
natural and convenient method of arranging them. Our obligation to 
that naturalist is, however, further enhanced by his lucid investigation 
of the pteropodous or wing-footed mollusks. Before his time the exist- 
ence of these prolific little animals was comparatively unknown ; Cavier 
was indeed the first to treat of them with any degree of scientific accu- 
racy ; observations were added by De Blainville, involving much import- 
ant information ; a good practical history of their habits and geographical 
distribution has been contributed by D’Orbigny; and we believe that 
M. Souleyet, a skilful demonstrator at Paris, is at this moment vigorously 
occupied upon the comparative anatomy of the species. 
The Pteropoda constitute a series of small but well-organized mol- 
lusks, inhabiting the great body of the ocean; they are said to dwell in 
swarms at different depths, and exist in greater or less abundance in all 
pelagic waters. Like the Carinarie they generally swim in numbers 
