PELECYPODA. 41 



maritime countries, and large quantities are brought to the London 

 market from the Dutch coast. At times they are unwholesome; 

 but all the exact causes of this are not known. Mussels seem to 

 be found on every shore, and some of the species are very widely 

 distributed — the common edible Mussel, M. edulis, being found on 

 every European coast, on the shores of North and South America, 

 in the Arctic and Antarctic Oceans, and probably on the coasts of 

 Australia. 



One group of Mussels (Lithodomus) burrow in rocks and other 

 shells, forming holes just large enough to contain their shells. 

 L. dactylus is sold as an article of diet on the shores of the Medi- 

 terranean. 



The Trigoniidce are one of those families which have all but dis- [Case 

 appeared during our period. Only three or four living species are E 'J 

 known, whilst more than a hundred fossil forms have been described 

 from the Jurassic and Cretaceous formations. Australia, where 

 some of the oldest types of animal life persist, furnishes also the 

 existing species of Trigonia. The animals have a long, sharply-bent, 

 pointed foot, like the Cockles, with which they make surprising 

 leaps. The shells are beautifully pearly within, and ribbed and 

 noduled exteriorly. 



The Arcidte are a family of strong ponderous shells varying [Case 23, 

 much in form and sculpture. The animals have a longish pointed E_H -] 

 foot, deeply grooved along the bottom, no labial palpi, and free 

 margins to the mantle, which are not prolonged into breathing- 

 siphons. Many of the Arks often anchor themselves by means of 

 a strong byssus. The shells of this family are usually radiately 

 ridged; and the hinge is composed of a number of teeth arranged 

 along the hinge-line, which is generally straight. Area tortuosa, 

 from China, has the valves curiously twisted. The section Bar- 

 batia is remarkable for the coarse fibrous character of the epidermis ; 

 Scapharca for its unequal valves ; and Cucullcea, from the Indian 

 Ocean, for the elevated ridge bounding the posterior muscular 

 impression. Pectunculus has the hinge-teeth arranged in an arched 

 series, and the shells are more regular in growth than in many 

 other forms of Arcida. 



The next large family, the Aviculida, includes Pinna, the [Case 24 

 " Wing-shells" {Avicula), the "Pearl" and "Hammer Oysters" A " H ^ 



