MOLLUSCA. rw 
compose the surface or exterior crust of our globe than the remains of 
elephants, hyppopotami, and whales.” 
Viewing this group as a class, D’Orbigny divides it into six orders, each 
of which, excepting the first, is subdivided into two families. Of the recent 
species, 68 genera and 900 species are known, more than half of which 
belong to the warm seas. D’Orbigny’s opinion of their classification is as 
follows: Though less complicated in their organization than many zoophyta, 
they have not a common life, nor are they aggregated like them, whilst 
their locomotive powers and testaceous envelope place them much higher. 
On the other hand they are in all respects less complete than the echino- 
dermata; and judging from the radiation of their filaments, the position of 
the foraminifera is in Cuvier’s radiated division, between the Echinodermata 
and the Zoophyta, as a distinct class. 
Oxper 2. Preroropa. This order takes its name from two large 
expansions, one on each side anteriorly, which are analogous to the wings 
of a butterfly both in appearance and action, being used in locomotion. 
This order was considered by Lamarck and Cuvier to form a distinct class, 
but Blainville and other distinguished naturalists are of opinion that its 
contents are essentially gasteropoda. Some species, as Clio borealis (pl. 76, 
jig. 78, one inch long), are naked; others, as Hyalwa, are provided with a 
delicate shell. These molluscs are monoicous, and swim about in the sea 
without the power of creeping or attaching themselves to solid bodies. 
This order contains the families Hyalecdw, Clicide, and Phillirhoide. 
In Hyaleea the branchiz are composed of pectiniform transverse laminz or 
leaves,* a character which would afford a name for the order in consonance 
with those of the allied orders, because Cuvier’s name, /eteropoda, apper- 
tains to the group as a class. 
Fam. 1. Hyaleide. The shell of Hyalea has a delicate texture and 
curious form, bearing a distant resemblance to that of Terebratula, to 
which the genus was at one time referred. The shell which covers the 
abdomen is translucent, subglobular, and has the appearance of a bivalve in 
which the two valves unite in a single piece without a hinge. The mollusc 
has two stout tentacles and two lips, and the wing-like expansions are placed 
near the mouth. They live in the high seas, seldom approaching the shores, 
flapping themselves along with great facility, and descending when dis- 
turbed. Being extremely abundant where they occur, they are devoured 
in great numbers by whales and other animals. Cleodora and Limacina 
are other genera. 
Fam. 2. Clioide. Clio has six retractile tentacles in two groups, and a 
rudimentary foot. Clio borealis is abundant in the north polar regions, 
where it is eaten by whales and fishes. 
Fam. 3. Phillirhoide. The head of Phylliroé is lengthened, and bears 
two tentacles, the snout is retractile, and the body is gelatinous and trans- 
parent, very much compressed laterally, and provided with a caudal fin. 
According to Peron and Lesueur the branchie are internal, and in the form 
* The group which Latreille named Phyllobranchia, had already been provided with a 
name. 
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