~ GENUS XIII. ARCA. 73 



tip, by which it holds itself to the rocks ; it is short, not ex- 

 tending above two lines beyond the shell. 



LiniiEBus has described 17 species, Gmelin has added 25, 



and Dr. Turton 1. Total, 43. 



Lamarck has divided this genus into four, as follow : 



NucuLA. Shell nearly triangular or oblong inequilateral ; 

 the hinge on a line, bent at an angle, furnished with 

 numerous transverse and parallel teeth ; a principal tooth 

 oblique and out of the row ; the beaks approximate and 

 timied backwards (see Area Nucleus, f. 36). 



Pectuxculus. Shell orbicular, nearly equilateral ; the hinge 

 in a curved line, with numerous teeth, oblique and in- 

 serted ; cartilage external (see Area Pectiinculus,f. 37). 



Arca. Shell transverse and inequilateral, the beaks distant ; 

 the hinge with many teeth in a straight line, transverse, 

 parallel, and inserted ; cartilage external (see Area Noee, 

 f. 38). 



CucvLL^A. Shell ventricose, nearly transverse, inequilateral ; 

 beaks distant ; the hinge in a straight line, with many 

 teeth set transverse and inserted, terminated at each end 

 by two or three parallel to the side ; cartilage external 

 (see Arca eueullata, Chemn. vol. 7. t. 33, f. 526, 528)u 



In consequence of these differences it would perhaps be 

 better to form four divisions in this genus, than to divide it 

 into four distinct g-enera. 



