FAMILY 9. CARDIACEA. 97 



The Cardiacea constitute a very natural group, although there are two 

 genera, Cardita and Cypricardia, which we have not included without 

 some hesitation, on account of their affinity with the Chamacea. The 

 shell of the Cardiacea is however somewhat peculiar in its characters, and 

 may be easily recognised. It is described as being equivalve, cordiform 

 or transverse, sometimes a little gaping ; and the valves are mostly cha- 

 racterized by being disposed in longitudinal ribs diverging in symmetrical 

 order from the umbones to the margin. There are generally two cardi- 

 nal teeth, irregular both in their form and situation ; and one or two 

 lateral teeth, but sometimes none. The family is divided into four 

 genera, as follows : 



Cardium. Cardita. 



Isocardia. Cypricardia. 



CARDIUM, Linnaeus. 



Testa aequivalvis, subinaequilateralis, subcordata, interdum postice hians; 

 ssepissime costis ab umbonibus ad marginem symmetrice divergen- 

 tibus ; marginibus valvarum crenulatis, interdentatis. Cardo, den- 

 tibus in utraque valva duobus approximatis, obliquis, confertim in- 

 tersertis ; lateralibus duobus remotis. Impressio muscularis pallii 

 integra. Ligamentum externum. 



The genus Cardium remains nearly as entire as in the time of Lin- 

 naeus, and is very numerous in species. The common Cockle, Cardium 

 edule, may be taken as the type of the series, their shells throughout 

 being of a more or less globular cordate form. 



The shell of Cardium is described as being equivalve, rather inequi- 

 lateral, more or less heart-shaped, and sometimes gaping on the posterior 

 side ; the valves are generally ribbed longitudinally, and the ribs, which 

 diverge in symmetrical order from the umbones to the margin, are often 



vol. i. o 



