179 



C. aspera, and C. avicularia, are the only species noticed by Lamarck, 

 as found fossil near Paris. 



CV. Isocardia. A heart-formed shell, with separated, involuted, and 

 diverging beaks. The hinge formed by two flattened cardinal inserted 

 teeth, and an isolated lateral tooth under the cartilage slope. 



This genus includes the Chama cor, Linn. I am not acquainted with 

 any shells, decidedly of this genus, having been found in a fossil state ; 

 casts, however, of shells very similar, are frequently found, and have 

 been distinguished as bucardites. 



CVI. Cardium. An equivalved subcordated bivalve, the valves den- 

 tated or plicated on their internal margins. The hinge formed by mid- 

 dle and lateral teeth : the middle ones, two, oblique and approximating, 

 those in each valve, crucially receiving each other, by mutual insertion ; 

 those of the side, remote and inserted. 



The shells of this genus are strongly characterized by the teeth of their 

 hinge, and by the projection of their beaks ; the latter giving them a 

 cordiform appearance. They are generally ornamented with longitu- 

 dinal ridges, more or less prominent ; and frequently with striae, scales, 

 or spines. The marks of attachment are two in number, and are but 

 faint. 



Specimens of a cardium, resembling C. aolicum. Linn, figured by 

 Lister, Tab. 314, No. 150, are sometimes found among the interesting 

 fossils of the whetstone pits, completely silicized; and still retaining, 

 very distinctly, the striae, disposed longitudinally, in the fore part, and 

 transversely in the posterior part. 



Among the fossils of Plumstead is found a cardium, striated exactly in 

 the same two directions as the preceding ; but the striae, especially the 

 transverse ones, are much more minute and faint than those of C. <eoli- 

 cum; agreeing in this respect, and indeed in all its characters, with 

 C. discors, Lam. Plate XIII. Fig. 3, is a representation of the silicized 

 shell of Devonshire, resembling C. aolicum. 



In the Harwich or Essex cliff, a shell somewhat like to C. mtcricatum, 



