LAMELLIBRANCHIATA. 233 



Radiolites the shell is inversely conical, bi-conical, or cylin- 

 drical, with dissimilar valves. The upper valve is sometimes 

 flat, sometimes conical, and has a central umbo. In Caprina 

 (fig. 191) the valves of the shell are dissimilar, the fixed valve 

 being conical, whilst the free valve is oblique, or is spirally 

 rolled. The free valve is thick, and is " perforated by one or 

 more rows of flattened canals, radiating from the umbo, and 

 opening all round the margin" (Woodward). The cavity of 

 the free valve is sometimes chambered. 



FAM. 9. TRIDACNID^E. Shell equivalve ; ligament external ; 

 muscular impressions blended, sub-central. Animal attached 

 by a byssus, or free. Mantle-lobes extensively united ; pedal 

 aperture large ; siphonal orifices surrounded by a thickened 

 pallial border. Foot finger-like and byssiferous. The shell is 

 truncated in front, the surface ribbed, and the margins toothed. 

 The family contains the single genus Tridacna, which is not 

 known to have come into existence before the period of the 

 Miocene Tertiary. 



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

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

 valve. Mantle open in front ; siphons usually very short ; foot 

 large and sickle-shaped. The only two 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 dorsal margin (fig. 192), the margins crenated, 



Fig. 192. Cardium Hillanum. Upper Greensand. 



and the pallial line more or less indented. It is doubtful it 

 any true Cardium has been detected in the Silurian Rocks. 

 With the Devonian, however, the genus begins to be well re- 

 presented, and it has continued up to the present day, attaining 

 its maximum in existing seas. The species figured above is 

 separated from the true Cockles by having the posterior slope 

 of the shell radiately striated, whilst the sides are concentri- 

 cally furrowed. 



