FAMILY 5. OSTRACEA. 171 



PLACUNANOMIA, Broderip. 



Testa irregularis, subaequivalvis, complanata, marginem versus plicata, 

 nterne vitrea. Cardo costis duabus angustis, longitudine insequali- 

 bus, ad basem convergentibus in valva altera ; sulcis duobus liga- 

 mentiferis in altera. Valva inferior prope ad cardinem fissurata ; 

 fissura tendine adhaesionis subosseo, inter testae laminas inserto, im- 

 pleta. Impressio musculi in utraque valva subcentralis ; impressione 

 tendinis superaddita in valva superiore. Animal organo subosseo 

 affixum. 



There are few instances on record in the natural history of the Mollusca, 

 where an apparent gap in the system has been more accurately filled up 

 than by the genus under consideration ; uniting, by the semblance of its 

 characters, two particular genera which had not been previously associated 

 without considerable doubt. We are indebted to the molluscomania of 

 that indefatigable traveller Mr. Cuming, who may be said to have spent 

 half his life in diving and dredging, for the discovery of this remarkable 

 animal. It was found by him attached to dead bivalve shells and corals, 

 at the depth of eleven fathoms, in the Gulf of Dulce, province of Costa 

 Rica, Central America ; and it has been recently described for the first 

 time by Broderip, under the appropriate title of Placunanomia. Its shell, 

 as the name denotes, is indeed exactly intermediate between those of 

 Placuna and Anomia : like the former, the hinge consists of two divari- 

 cate ribs, bearing the ligament ; and, like the latter, there is an opening 

 in the lower valve for the passage of a bony tendon by which the animal 

 attaches itself. This subosseous organ, however, is rather a singular 

 modification of that in the Anomia : instead of directly perforating the 

 lower valve, it is first introduced, like a flat plate, between the lamina? of 

 which the valve is composed, and then passes out, for the purpose of 



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