LAMELLIBRANCHIATA. 



223 



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

 plied to the beds in question. 



Fig. 175. Plicatula placujiea, 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, 



220 



Fig. 176. Avicula deinissa, 

 Lower Silurian. 



Fig. 177. Ambonychia xpdiata, 

 Lower Silurian. 



Gervittia, Perna, Inoceramtis, and Pinna. In the genus Avicula 

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



