ARCA. PECTUNCULUS. NUCULA. NAYADA. 53 



Pectiuiculus Area. 



Nucnla Area. 



" The hinge of Area in this arrangement is 

 always straigJtt., furnished with a number of 

 teeth ; the ligament is external. The shells are 

 open at one end, for the animal throws out at the 

 aperture a number of threads, by which it^fastens 

 itself to the rocks. The species are thirty-seven, 

 and also several fossil. 



'' The orbicular form of Peetunenlus, and its 

 arched hinge, distinguish this genus from the pre- 

 ceding one. They are allied to the Pectens by 

 their form, and their crenulated internal margin. 



" The hinge of Nucula is set with little teeth 

 on each side, like a comb. It is pearly within, 

 and sometimes small pearls are found in the 

 shell. Peetunenlus costatus is found in London 

 clay. 



" Trigoniana is a small family containing Trl- 

 gonia and Castalia. The first is a fossil genus 

 chiefly. Some species are found in the Portland 

 stone, or oolite beds. 



" The next family contains the Nayada, 

 chiefly composed of fluviatic, or fresh-water 

 shells. They are covered with an olive-brown 

 epidermis, which is constantly found eroded, or 

 destroyed at the beaks. The muscular impres- 

 sions are lateral and much separated ; one of 



