462 



MOLLUSCA 



PHYLUM VI 



epidermis ; area amphidetic or ohscure ; ligament parivincular, usually opisthodetic and 

 external ; gills usually ßlibranchiate ; mantle lobes without ocelli, iiiore or less free^ 

 generally with the anal siphon complete and the hranchial incomplete ; foot small, digiti- 

 form, gi'ooved, hyssiferous ; monoecious ; mostly marine. 



Family 38. Modiolopsidae Fisclier (emend.). 



Shell modioliform, usually eqidvalve, ß'ee, thioi, with suh-equal adductor scars ; 

 ligament deep-seated ; hinge edentulous or dysodont ; sometimes hyssiferous. Ordovician 

 to Cretaceous. 



The lieavier fornis show an obtuse ridge or two extending from the beaks toward the basal 

 margin. The pedal scars are separate from and behind the anterior adductors. The forms 



included here aj)pear to be 

 the prototypes of the Myti- 

 lidae, from which they difFer 

 chiefly in those characters 

 that are common to most of 

 the ancient types, such as the 

 sub-equality of the adductor 

 scars and their more dorsal 

 Situation. The recent Idas is 

 very similar. 



Modiolopsis Hall (Fig. 

 726). Valves elongate- 

 oval, closed, with iiearly 

 terminal beaks, narrow 

 hinge plate, and edentulous 

 hinge, Ordovician and 

 Silurian. 



Modiomorpha Hall. 



Similar, but with a wider 

 hinge plate, and Single, 

 oblique, elongate, posterior 

 ridge - like tooth. De- 

 vonian. 



Myoconcha Sow. (Fig. 



727). Hinge usually with an elongate cardinal, and a long, weak, lateral tooth in 



the right valve ; otherwise resembling Modiolopsis. Carboni- 



ferous to Cretaeeous. 



(?) Hippopodium Sow. Thick, infiated ovate, concentric- 



ally waved. Hinge with a long, blunt, oblique, cardinal 



tooth, or edentulous ; adductor scars strong. Jura. 



Modiolodon, Whiteavesia, Eurymya, Aristerella, and Prolo- 

 hella Ulrich; Ordovician; North America. Goniophora 



Phillips. Silurian and ]3evonian. Orthonota Conrad. De- 



vonian. Orthodesma Hall and Whitf. Ordovician; North 



America. 



Family 39. Mytilidae Fleming. 



Shell equivalve, very inequilateral^ heteromyarian, slightly 

 gaping, typically dysodont ; area amphidetic or noite ; ligament 

 usually external^ deep-seated; rarely with an alivincular 

 internal resilium ; pallial line simple; mantle lohes united 

 helow the anal siphon, otherwise free; generally hyssiferous. 

 Devonian to Kecent. 



Pio. 726, 



Modiolopsis modiolaris 

 (Conrad). Ordovician ; Cin- 

 cinnati, Ohio. i/i. 



Fig. 727. 

 Myoconclia striatula Goldf. 



Lower Oolite ; 

 Bayeux, Calvados, i/i- 



Fig. 728. 



Mytilus suhlaevis Sow. Great 

 Oolite ; Minchinhampton, Eng- 

 land, i/i- 



