BIVALVIA. 151 



Cardium,* Linnaus. 



Pectuncultjs. Adanson, 1757. 

 Cerastes and Cerastoderma. Poli, 1795. 

 IsocAKDiA, (sp.) Klein, 1753. 

 Cardissa. Megerle, 1811. 

 Aphrodita. Lea, 1833. 

 LuNULACARDiUM. Munst., 1840. 

 LiEviCARDiUM. Swains, 1840. 

 Hemicardium. Id. 



ACARDO. Id. 



Papyridea. Id. 



Serripes. Beck, sec. Gould. 



MoNODACNA (part) and Didacna. Eichw., 1841. 



Generic Character. Shell equivalve, sub-equilateral, more or less heart-shaped; 

 generally closed, sometimes gaping posteriorly: usually inflated, orbicular, or ovate, 

 costated, costse variable, from nearly obsolete to large and prominent, sharp, round and 

 naked, often ornamented vi^ith scales or tubercles ; margin dentated or crenulated ; 

 hinge composed of two cardinal teeth in each valve, and two remote and prominent 

 lateral teeth. Ligament external. Impression of the mantle without a sinus. 



Animal of the general form of the shell, its mantle open in front, with the margins 

 generally plain, occasionally fringed, particularly towards the posterior, and around 

 the syphons ; these are short, and slightly separated, with the margins of one or both 

 always fringed ; foot large, sub-cylindrical, and bent at nearly a right angle, possessing 

 the form of an inflected arm or elbow-joint, 



Eichwald has described some species from the Caspian sea, which he has separated 

 into three genera, under the names of Didacna, Monodacna, and Adacna, depending for 

 his characters upon the numerical presence, or the absence of the hinge-denticles. The 

 animals of the latter division appear deserving of separation, being furnished with 

 elongated syphons, and consequent indenture of the mantle mark. Some fossils 

 also from the Palaeozoic rocks, justly erected into a Genus by Professor Phillips under 

 the name PleurorJii/ncJius, much resembled the general character of cockles, but were 

 probably furnished . with elongated syphons, as the shell is greatly produced on the 

 posterior side. Species possessing undoubted characters of this Genus, have been 

 obtained from the Middle Secondary Formations, and they are largely developed 

 in the Tertiaries, while from the present seas not less than 200 species have been 

 obtained. Their range geographically takes in nearly the whole surface of the Globe, 

 and they are met with in Estuaries as high as low-water mark, while some are 

 inhabitants of the sea at depths not exceeded by any other Molluscs. They generally 

 frequent sand or sandy-mud bottom, where they often congregate in prodigious numbers. 



* Etym. naphia, tlie heart. 



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