526 TRANSACTIONS OF THE WAGNER FREE 



FAMILY DIMYID,. 



Shell inequivalve, irregular, closed, auricles not differentiated, ostreiform, 

 dimyarian, sessile ; shell substance subnacreous and fibrous ; area amphidetic, 

 obscure ; ligament obsolete, resilium alivincular, internal ; hinge armature 

 taxodont, obsolete ; crural development feeble ; gills filibranchiate, the stem 

 distally attached to the mantle ; the inner direct filaments wanting, the outer 

 filaments not reflected ; foot and byssus absent ; mantle lobes free with the 

 margin papillose, with a curtain but without ocelli ; anterior adductor distinct, 

 small, posterior duplex, larger ; palpi obsolete ; anal end of the rectum free, 

 erectile. 



Tertiary to recent fauna. 



Ex. Dimya (+Marg-ariona), Dimyodon. 



I have already noted my impression that Dimya is, on the whole, rather 

 a degenerate than an archaic form ; but it is difficult to indicate which of the 

 other families is most nearly related to it. 



FAMILY LIMID^E. 



Shell equivalve, auriculate, gaping, pectiniform, monomyarian ; shell sub- 

 stance fibrous, with minute tubules, not nacreous or prismatic ; hinge edentu- 

 lous, or with traces of taxodont armature ; area amphidetic, equal in both 

 valves; ligament alivincular, resilium subinternal ; gills filibranchiate, with di- 

 rect and reflected limbs; ganglia normal, mantle lobes free, with profuse retrac- 

 tile tentacular processes, ocelli and a curtain; foot small, digitiform, grooved, 

 usually byssiferous, the byssus passing through the gape of the valves ; anal 

 end of the rectum free, erectile. 



Carboniferous to recent fauna. 

 Ex. Plagiosloma, Lima, Litnalula, Linuza. 



These animals frequently spin a nest of byssal threads like Modiolaria, 

 but are capable of swimming vigorously by opening and closing the valves ; 

 in motion the beaks are posterior. The shells are colorless, the soft parts 

 usually bright colored. 



Superfamily ANOMIACEA. 



Shell monomyarian, not alate ; edentulous, or isodont, usually sessile ; 

 shell substance nacreous, tubuliferous, with traces of a prismatic layer ; area 

 obscure, usually small, amphidetic; ligament obscure, with an alivincular in- 

 ternal resilium ; ventricle free from the rectum, not enclosed in a pericardium, 

 but projecting in the pallial chamber, dorsal to the gut, with a single aorta ; 

 gills filibranchiate, with direct and reflected limbs, united to the mantle and to 

 each other; mantle lobes free, without ocelli; otocyst closed; foot small, 

 grooved, digitiform; anal end of rectum usually free, erectile; dioecious, 

 marine. 



