320 INTRODUCTION TO CONCHOLOGY. 



well-defined assemblage of cliaracters ; the teeth are small 

 and numerous^ set in a straight or curved line, and the 

 ligament is external, with the exception of the Nucula, in 

 which this appendage is internal. The teeth of one valve 

 closely interlock with those of the other. 



Tour genera pertain to this family — CucuU6ea, Area, 

 Tectunciilus, and Nucula. 



Lamarck instituted the genus CncnUaa on account of a 

 broad concamerated shelf in the anterior of the shell, for 

 the attachment of the anterior muscle; an area or facet 

 between the umboes, which is formed by the gradual 

 thickening of the dorsal edges, is also very characteristic 

 in its appearance. 



The Area likewise presents a distinct, accurately-defined 

 assemblage of generic characters, wliether as regards the 

 animal or shell, — ^peculiar, without doubt, because the um- 

 boes are, with rare exceptions, more or less widely sepa- 

 rated from each other, by the interposition of a large area 

 which sustains the ligament, deposited either over its entire 

 surface, or in superficial lozenge-shaped grooves, and this 

 often to a considerable extent. The hinge is composed of 

 a long rectilinear series of fine plate-Hke teeth, varying from 

 thirty and forty, to one hundred and twenty or more in 



