A MONOGRAPH 



OF THE 



MOLLUSCA FROM THE CRAG. 



BIVALVIA, LinruBus, 1767. 



BiVALViA. Bonanni, 1681. Lister, 1686. Flem. 1828. 



DiTHYRA. Arist. Turt. 1822. Swains. 18-10. 



DiTOMA. Tournefort, 1742. 



AcEPHALA Testacea (part). Cuv. 1789. 



Lamellibranchiata. Be Blainv. 1814. 



CoNCHiFERA (part). Lam. 1818. 



CoNCH.'E. Leach. 1819. 



Pelecypoda. Goldfuss, 1820. 



ExDOCEPHALA (part). Lat. 1825. 



Elatobranchia. Menke, 1830. 



CoRMOPODA. 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 branchice, 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 move, as it were, like a door upon its hinge, or rather 



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