VII 



MOLLUSCA SYSTEMATIC REVIEW 



15 



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

 Branchial filaments entirely free (Anomia, Placuna}. Fam. Arcidae : the branchial 

 filaments of eacli row connected by ciliated discs ; Dimyarian. Xo siphons. Foot 

 large (Arcfi, Pedunculus). Fam. Trigoniidse : ctenidia like those of the Arcidce; 



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

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

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



Dimyarian. No siphons (Trigonia}. Fam. Mytilidse (excluding Aviculidce) : cten- 

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

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

 [Fig. 22], Mod I' ilfi, Lithodomus [boring mussel], Modiolaria). 



ORDER 3. Pseudolamellibranchia. 



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

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



