476 



LAMELLIBRANCHIATA. 



eared, and the shell is covered with spines, foliaceous expan- 

 sions, or ribs radiating from the beak. The lower valve has a 

 triangular hinge-area, and there are two teeth in each valve. 

 The Spondyli seem to have commenced in the Jurassic 

 period, are abundant in the Cretaceous, and have continued 

 through the Tertiary period to the present day. 



Lastly, the Plicatulce (fig. 333) approach the Spondyli 

 nearly, by having an inequi valve shell, which is attached 

 by the right valve, and by having two hinge-teeth in each 

 valve, The shell, however, is rarely eared, the hinge-area is 

 obscure, and the valves are not spiny, though they may be 

 plaited. The Plicatulce extend from the Trias to the present 

 day, and they abound to such an extent in parts of the 

 Lower Greensand (Cretaceous), as to have given rise to the 

 name of " Argile a Plicatules " applied to the beds in 

 question. 



Fig. 333. Plicatula placunea. Lower Greensand. 



FAM. 2. AVICULID^:. Shell inequivalve, very oblique, 

 attached by a byssus : hinge nearly or quite edentulous. 

 Mantle -lobes free; pallial line entire. Anterior adductor 

 small, leaving its impression within the umbo ; posterior 

 adductor large and sub-central. Foot small. 



The family of the Aviculidce is a very large one, and has 

 a most extensive development in past time, beginning under 

 various types in the Lower Silurian, and being continued 

 thereafter to the present day. The various types included 

 in this family may be briefly considered under the following 

 groups : 



Firstly, we have the great group of shells of which 

 Avicula itself (fig. 334) is the type. In this genus the shell 

 is oblique and very inequivalve. The right valve has a 



