VII 



MOLLUSGA— SYSTEMATIC REVIEW 



15 



without siphons ; Monorayarian. Foot small ; body and shell asymmetrical. Attached. 

 Branchial filaments entirely free (^Momia, Placvna). Fam. Arcidae : the branchial 

 filaments of each row connected by ciliated discs ; Dimyarian. No siphons. Foot 

 large (Area., Pectuncuhis). Fam. Trigoniidse : ctenidia like those of the Ardda: ; 



Pig. 22.— Mytilus edulis (after Meyer and Mobius), left aspect, with extended foot attaching a 

 byssus thread ; d, byssua threads ; a, exhalent aperture (anal siphon) ; !>, fringed edge of the in- 

 halent mantle aperture ; c, object to which the animal is attached. 



Dimyarian. No siphons ( Trigonia). Fam. MytilidsB (excluding Aviculidcs) : cten- 

 idia connected by means of non - vascularised trabeculse. The anterior adductor 

 is smaller than the posterior (Heteromyarian). With siphons. Foot long. (Mytilus 

 [Fig. 22], Modiola, Lithodomus [boring mussel], Modiolaria). 



Ordek 3. Pseudolamellibranohia. 



The consecutive ctenidial filaments of each row are connected by means either 

 of ciliated discs or of vascularised trabecule ; and the ascending and descending 



