TRIGONIA. MUSSELS. 79 



on each face (jig. 99), 

 and entering between 

 four plates of the left 

 (opposite) valve, like- 

 wise crenulated upon 

 their internal face 

 only. There is but 

 one living species 

 known, Trigonia 

 pectinata, which in- 

 habits the seas of Figt 99._ TR1GON iA PECTINATA. 

 New Holland ; exter- 

 nally, it has the aspect of Pecten without ears. 



21. In the Family of MYTILACEA, the mantle is widely open 

 in front, as in the Ostracea, but there is a particular opening for 

 the escape of excrement ; this orifice, however, is not prolonged 

 into a tube, as in the families that follow, and there is no special 

 aperture for the passage of water for the purpose of respiration. 

 There is always a distinct foot, and the valves are approxi- 

 mated by the action of two adductor muscles. 



The Mytilacea? resemble each other sufficiently to be commonly 

 known under the single name of Mussels, but they require, never- 

 theless, to be separated into several genera, the most important 

 of which are, Mussels properly so called, Mytilus, Anodonta, 

 and Unio. 



22. MUSSELS PROPERLY so CALLED Mytilus abound on the 

 rocks of the coast of France and of other countries, where they 

 live fixed by the byssus, and generally closely united to each 

 other. Their shell is shut, and its valves, triangular in form, 

 are equal, arched and united by a narrow ligament on the side of 

 their acute angle. The mouth of the animal is situate near 

 the summit of the shell, the opposite extremity of which gives 

 passage to the byssus ; the anus also is placed near the hinge ; 

 and opposite to this orifice there is a particular opening or small 

 tube formed by the mantle; towards the round angle of the shell, 

 where the water necessary for respiration passes, the edge of 

 the mantle is fringed ; and the foot is slender, cylindrical, and 

 furnished behind with a silky byssus. Mussels are generally 

 eaten as food, but they sometimes occasion a kind of poisoning, 

 accompanied by very violent symptoms, occasionally followed 

 by death. 



21. What are the characters of the family of Mytila'cea ? 



22. What are the characters of the genus My'tilus ? (From the Greek, 

 mutilos, a mussel.) 



