222 MOLLUSCA. 



the shell rests upon the right valve, and the beaks are furnished 

 with ears. The anterior ears are usually the largest and most 

 prominent, and the shell is generally furnished with ribs radiat- 

 ing from the umbos. The right valve is the deepest, and is 

 notched below the anterior ear. Counting in its sub-genera, 

 nearly five hundred species of the genus Pecten are known in 

 the fossil state, commencing in the Devonian Rocks, and ex- 

 tending to the present day. The Palaeozoic Pectens are dis- 

 tinguished from their successors in more modern rocks by 

 having the posterior ears larger than the anterior; and they 

 are therefore placed by M'Coy in a new genus, under the 

 name of Aviculopecten. They are, however, usually regarded as 

 belonging to the Aviculida. 



In the genus Lima (or Plagiostoma) the shell is equivalve and 

 free, whilst the beaks are separated from one another and are 

 eared. The genus is represented by numerous species in the 

 rocks of the Secondary period, and has survived to the present 

 day. 



In Spondylus (fig. 174) the shell is inequivalve, and is fixed by 

 the right valve to some foreign body. The beaks are apart and 



Fig. ^^.Spoiidylus spinosus, Chalk. 



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 Cretaceous 

 period, in which they are very abundant, and they have con- 

 tinued through the Tertiary period to the present day. 



Lastly, the Plicatula (fig. 175) approach the Spondyli nearly, 

 by having an inequivalve 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 Plica- 

 tula extend from the Trias to the present day, and they abound 

 to such an extent in parts of the Lower Greensand (Cretaceous), 



