46 



having a ventral opening for the passage of the foot, and a double opening 

 behind for the purposes of inhaling and excretion, of which the margins 

 are in many genera prolonged into siphons. 



The shells of the Lamellibranchiates, present forms and varieties of the 

 highest interest to the conchologist, and, as in other genera of mollusks, 

 there are often curious differences of sculpture and colour where there is 

 little apparent specific difference in the animal. The most striking modi- 

 fications of character are those of the ligament and the hinge. Changes 

 in the development of these are for the most part accompanied by corre- 

 sponding changes in the objects generally, and they have been universally 

 selected for the determination of genera. The class may be divided into 

 orders according to the number of the internal adductor muscles : — 



Unimusculosa. Bimuscclosa. 



Order 1. UNIMUSCULOSA. 



Shell ; attached to the animal by a single central muscle. 



In this division, comprising the Oysters, Scallops, Mussels, and Clams, 

 the shell is attached to the animal by a strong central muscle, accompanied 

 in some instances by one or more small collateral muscles. Any one who 

 has watched the oyster-vendor at his stall may have observed the force 

 which is required to detach the animal from its shell, when cutting at the 

 muscle. Many a Lamellibranchiate of small size and of comparatively 

 frail texture possesses a massive shell, and nothing short of great strength 

 in the adductor muscle would serve for closing it. The Order includes 

 five families, as follows : — 



ostracea. mytilacea. aviculacea. 



Pectinacea. Tridacnacea. 



Family 1. OSTRACEA. 



Shell ; mostly attached to foreign substances, irregular, foliated 

 or laminated; right valve generally larger than the left valve. 



In this family the shells are of an irregular growth, with only a faintly- 

 developed hinge, and the species are not very readdy definable. There is 

 considerable apparent difference in the extreme forms of the group, the 

 Anomia and the Ostrea, but they are connected by two intermediate links', 

 Plamnanomia and Placiina, in which their peculiarities are very naturally 

 blended. The shell of Anomia is toothless, and the animal attaches itself 



