LAMELLIBRANCHIATA. 531 



the above-cited organs are formed, is peritoneum rather than pleura. 

 The hinge of the valves is the unseparated, but modified, remnant of 

 the germ-mass, which connects together the two individuals resulting 

 from its incomplete fission : it is much more like the common pedicle 

 or base of the compound Ascidian, than the " spina dorsi " of a verte- 

 brate animal. 



Thus we learn how valuable are the facts derived from the study 

 of the progressive building up of the animal frame. In the case of 

 the chiss which has this day occupied our attention, a knowledge of 

 their development, partial and imperfect as it must be confessed to 

 be, has nevertheless thrown new light on the peculiar organisation 

 of the mature animal, has rectified misconception of the nature and 

 homologues of their parts, and may have suggested truer theories of 

 some of their more remarkable living habits. 



Class LAMELLIBRANCHIATA. 



Body with a dextro-sinistral bivalve shell, lined by a commonly 

 more or less closed mantle. Oral tentacles four ; branchiaj lamelli- 

 form, usually in two pairs. 



A. MoNOMYARiA. With one (the posterior) adductor. 

 Family Ostreadce. 



Lobes of the mantle widely separated ; foot small, byssiferous, or 

 absent. Shell inequivalve, slightly inequilateral ; hinge usually eden- 

 tulous. 



Genera Ostrea, Anoinia, Placu7ia, Pecten, Lima, Spondylus, 

 Pedum, Plicatula. 



B. DiMYAEiA. With two adductors. 



Family AviculidcB. 



Lobes of the mantle widely separated; foot small, byssiferous; 

 anterior adductor very small, leaving its impression within the umbo. 



Genera Avicula, Meleagrina, Malleus, Vulsella, Gervillia, Perna, 

 Crenatula, Inoceramus, Pinna. 



Family Mytilidce. 



Mantle-lobes united between the branchial and anal slits or orifices; 

 foot cylindrical, byssiferous. Shell equivalve ; hinge edentulous. 



Genera Mytilus, Myalina, Modiola, Lithodomus, Crenella, Dreis- 

 sena. 



MM 2 



