82 



CUCULL^A. 



arca cucullus. — Linn. 



Sowerbys Genera, No. 4, pi. 6, Jig. 1, 2, 3. 



This genus very nearly resembles the G. Arca of Lin- 

 naeus, from which La Marck has separated it, in con- 

 sequence of several manifest distinctions of structure. 

 The shape is more gibbous or trapeziform ; anterior side 

 truncated obliquely, and the hinge (which in young 

 shells is similar to that of the Arca) by growth or age 

 becomes displaced, or appears obsolete, exhibiting pa- 

 rallel ribs, which terminate it, and give the teeth a more 

 horizontal appearance than in the G. Arca, as it now 

 stands defined by LaMarck; another very remark- 

 able distinction is the muscular impression within, to 

 one side of which is an ear-shaped testaceous appen- 

 dage, placed at an angle with the anterior surface, and 

 forming a chamber or division. Shell equivalve, ine- 

 quilateral, rhomboidal, heart-shaped, truncated at one 

 end, and very globose; umbones distant and separated 

 by the angular groove or area of the ligament, which 

 is altogether external. Hinge linear, straight, with 

 small transverse teeth, having at its extremity from 

 two to five parallel ribs. 



