A MONOGRAPH , 
OF THE 
MOLLUSCA FROM THE CRAG. 
BIVALVIA, Zinneus, 1767. 
Bivatvia. Bonanni, 1681. Lister, 1686. Flem. 1828. 
Dirnyra. Arist. Turt. 1822. Swains. 1840. 
Dirroma. Tournefort, 1742. 
AcrpHaLa TESTACEA (part). Cuv. 1789. 
LAMELLIBRANCHIATA. De Blainv. 1814. 
Concutrera (part). Zam. 1818. 
Concu®. Leach. 1819. 
PrLtecypopa. Goldfuss, 1820. 
Enpocepnuata (part). Lat. 1825. 
ExvatoprancHia. Menke, 1830. 
Cormopopa. Burm. fide Herrm. 
ANIMALS of this Division of the Linnean Testacea have no proper head, their most 
vital parts are enveloped in a mantle, or pallium, as it is called, which surrounds them 
on all sides; the edges of this mantle are sometimes plain, at others fringed, and are 
more or less united: what is called the foot, is generally a large and powerful muscular 
mass, capable, in some species, of being protruded beyond the shell to a considerable 
distance. The respiratory organs, or dranchie, are usually four in number, and are 
arranged in the form of ruffles enveloping the abdominal mass, but entirely included 
within the mantle ; in some few of the species, the number of these feathery append- 
ages is less, while in others there are more, than four. The animal is protected by 
two portions of shelly matter called valves, these are secreted by and formed upon the 
mantle, and are articulated together by a cartilage and ligament, by which the two 
pieces are held in position, and moye, as it were, like a door upon its hinge, or rather 
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