214 Monograph of the LamelUhranchiata. 



yellow filameiits extend to the mouth, where anothei' pair of ganglia 

 supply the tentacles, and give off a delicate ring round the oesophagus. 

 In the Diinyaria the main pair of ganglia (branchial) are at the under 

 edge of the posterioi* adductor, sending, as before, cords to the supra- 

 03sophageal ganglia, between the mouth and the anterior adductor ; 

 numerous other small ganglia are developed in various positions. 

 Healing — Cycla^ has at the anterior part of the labial tentacles a cavity, 

 with a cretaceous, oscillating nucleus, considered by Dr. Siebold to be 

 the ear. Sight — Peden has numerous small eyes round the edge of 

 the mantle, but most of the types are blind. Developnent — the 

 spermatic filaments discharged from the males are drawn in by the 

 branchial currents of the females. The young, or embryo, of the 

 dimyarian genus Anodon, have only a single adductor, as in the 

 monoiHT/aria, and possess a byssus. The shells are extra-vascular, of an 

 external cellular or fibrous and a thin internal nacreous layer, form- 

 ing the hinge teeth. ' . 



This class is divided into three orders: 1st. Pletiroconcha ; 2d. 

 Isedrolotila ; 3d. MacrotYachia. 



Order Pleuroconcha — (D'Orb) . 



Shell inequivalve, unsymmetrical. Animal unsymmetrical ; lobes 

 of the mantle disunited, never forming siphonal tubes ; pallial impres- 

 sion consequently entire. Natural position — resting on the side. 



They are fixed to foreign bodies, either by a byssus or by the sub- 

 stance of the lower valve ; the shells in the latter case being remarka- 

 ble for the irregularity of their growth. 



This order contains the following families: 1st. Anomiida- ; 2d. 

 Ostreidce; od. Etheridcc ; 4th. Chamidce; 5th. Spondylidie ; 6th. 

 Pectinictce; 7th. Aviculidce, etc. 



The family Avicididce (wing shells) is described as follows : 



Shell inequivalve, very oblique, resting on the smaller (right) 

 valve, and attached by a byssus ; epidermis indistinct ; outer layer 

 prismatic, cellular ; interior, nacreous ; posterior muscular imj)ressiou 

 large, subcentral, anterior small, within the umbo ; pallial line irreg- 

 ularly dotted ; hinge line straight, elongated ; umbones anterior, 

 eared, the posterior ear wing-like ; cartilage contained in one or several 

 gi-ooves ; hinge edentulous, or obscurely toothed. 



The genus Avkula belongs to this family. 



Order, Isedrolotila — (McCoy). 

 Shell equivalve, symmetrical ; animal, having the mantle usually 



