258 LECTURES ON MOLLUSCA. 



very short, siphons, and a mantle open in front. It has fewer hinge- 

 teeth, and has been associated with Gouldia, which probably belongs, 

 with the true Circes, to the Astartids. 



Family Isocardiadje. {Heart Cocldes.) 



The animal of Isocardia, like that of Cyprina, has short pipes and 

 open mantle. The shell is swollen, allowing of a very large gill-cavity ; 

 and the beaks spirally twisted, with the hinge-teeth following the curve 

 of the margin. The foot is small, for sand burrowing. The fossil 

 species are very numerous ; but many called by this name belong to 

 the Pholadomya group, and some to the Areas. In the little group 

 Cardilia, the ligament is fixed on an internal plate, as in some of the 

 Lucinids. 



Family Cardiadte. (Cockles.) 



The Cockles abound in shallow water, in almost all sandy bays, 

 and are extensively collected for food. On the northern shores of 

 the Atlantic States, they are curiously rare; their place in the market 

 being supplied by the clams. The animal has short pipes, covered 

 with feelers; and open mantle, generally plaited at the margins. Most 

 of the bulk of this mollusk consists of the foot, which is long and 

 knee-shaped, doubled up into the gill-cavity when at rest, used as a 

 leaping-pole when extended. The typical species of Cardium have 

 swollen shells, with radiating ribs interlocking at the margins. The 

 hinge teeth are small, but, with the side teeth, are deeply interlock- 

 ing. The shells of Bucardinm gape at the sides; those of Levicar- 

 dium are smooth outside, but generally toothed at the margins ; those 

 of the boreal Serripes are almost edentulous. The cretaceous form 

 Protocardium has the bulk of the shell concentrically furrowed, while 

 the side has the usual radiating furrows. The Hemicardium group 

 are keeled and flattened on one side; while the abnormal and very 

 beautiful Cardissa group are flattened out on each side, with a hollow 

 projecting keel. Papyridea is like a thin Bucardium, flattened in 

 the opposite direction from Cardissa, and very much produced on one 

 side. 



The very aberrant Cockles of the Caspian Sea have very long pipes, 

 not fringed, and united nearly to the ends. The foot is shaped as in 

 Venus. The shells are shaped like common Cockles, but without 

 teeth. Sometimes however there are one or two small ones. They 

 are called Adacna, (with Monodacna and Didacna,) and are often ar- 

 ranged with the Pholadomyas. 



Shells having a general resemblance to Cockles have been found 

 fossil in all strata, beginning from the Upper Silurians. Several 

 however must have had very different animals. The ancient group 

 Conocardium, is like Hemicardium with a very long tube projecting 

 from the truncated side, like the wing of an Avicula. The structure 

 of the shell also is in cubical prisms ; but the tube was probably for 

 the protection of Adacnoid pipes, as in the Gastrochcenids. 



Family Astartidje. 

 The shells of this very extensive family partake of the characters of 



