LAMELLIBRANCHIATA. 



223 



as to have given rise to the name of " Argile a Plicatules " ap- 

 plied to the beds in question. 



Fig. 175. Plicatula placunea, Lower Greensand. 



FAM. 2. AVICULID^E. Shell inequivalve, very oblique, at- 

 tached by a byssus ; hinge nearly or quite edentulous. Mantle- 

 lobes free; anterior adductor small, leaving its impression 

 within the umbo; posterior adductor large and sub-central. 

 Foot small. 



Leaving out of consideration the genus Aviculopecten, which 

 holds a dubious position, and has been already spoken of, the 

 chief fossil genera of the Aviculidce are Avicula, Posidonomya, 



Fig. 176. A vicula demissa, 

 Lower Silurian. 



Fig. 177. A mbonychia. radiata, 

 Lower Silurian. 



Gervillia, Perna, Inoceramus, and Pinna. In the genus Avicula 

 (fig. 176) the shell is oblique and very inequivalve. The right 



