496 



LAMELLIBRANCHIATA. 



shell is closed, and the pedal aperture is only indicated by 

 small serrations of the margin of the valves. Tridacna is 

 only known in a fossil condition by a few species from the 



Fig. 361. Tridacna -media. Tertiary. Interior of right valve. 



later Tertiaries. The Palaeozoic Eurydesma has also been 

 referred to this family. 



FAM. 10. CARDIAD^E. Shell equivalve, heart-shaped, with 

 radiating ribs ; cardinal teeth 2 ; lateral teeth 1-1 in each 

 valve, the former crossing each other when the shell is 

 closed. Mantle open in front ; ligament external ; siphons 

 usually very short; foot large and sickle -shaped. The 

 two principal genera of this family are Cardium and 

 Conocardium. 



In Cardium are comprised the true Cockles, in which the 

 shell is ventricose, the beaks pronounced, and placed nearly 



in the centre of the dor- 

 sal margin, the margins 

 crenated, and the pallial 

 line more or less in- 

 dented. It is doubtful 

 if any true Cardium has 

 been detected in the 

 older Palaeozoic rocks. 

 With the Carboniferous, 

 however, the genus be- 

 gins to be represented, and it has continued up to the present 

 day, attaining its maximum in existing seas. Papyridea, 

 Lcevicardium, and LitJiocardium are Secondary to Eecent 

 types closely allied to Cardium proper. Protocardium (fig. 



Fig. 3t>2. Vardium (Protocardium) Hillanum. 

 Upper Greensand. 



