72 



CHAPTER XVI. 



OF THE ARCA. GENUS XIII. 



ARCA. 



"Animal a Tethj/s ? Shell bivalve ; valves equal. The hinge with nume- 

 rous acute teeth, alternate and inserted." — Linn. 



The shells of this genus are easily disting-uished by the nume- 

 rous small teeth which are arranged in a line (/. 38) ; which 

 in some species is straight, as in Area Noce ; in others it is bent 

 like an arch, as in Area Peetuneulus (J. 37) ; and sometimes 

 is bent like a broken line, as in Area Nucleus (/. 36). . The 

 different species vary much in shape : some approach towards 

 a boat-like form, as in Area Noce ; these have the beaks sepa- 

 rated to a considerable distance by a flattish space somewhat 

 lozenge-shaped ; some approach to the shape of a Cockle, as 

 Area Peetuneulus. The cartilage is external in all. 



The animal appears to be a species of Cockle, protruding 

 two short tubes from one end of the shell, and a scythe-shaped 

 foot at the front ; but the animal of Area Noa seems to vary 

 considerably fi'om the rest ; the margin of the valves opposite 

 the hinge is a little hollowed, so that when shut together 

 they leave an opening ; the foot protrudes at that part, and is 

 somewhat like the foot of the Muscle * ; it ends in a flat horny 



* See Adanson, p. 251. 



