462 



MOLLUSCA 



PHYLUM VI 



epidermis; area amphidetic or obscure; ligament iHirivincular, usually opisthodetic and 

 external; gills usually filihranchiate ; mantle lobes without ocelli, more or less free, 

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

 form., grooved, byssiferous ; monoecious ; mostly marine. 



Family 38. Modiolopsidae Fischer (emend.). 



Shell modioliform, usually equivalve, free, thin, with sub-equal adductor scars; 

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

 to Cretaceous. 



The heavier forms show an obtuse ridge or two extending from the beaks toward tlie basal 

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



included here appear 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 nearly 

 terminal beaks, narrow 

 hinge plate, and edentulous 

 hinge. Ordovician and 

 Silurian. 



Modiomorpha Hall. 



Similar, but with a wider 

 hinge j^late, and single, 

 oblique, elongate, posterior 

 ridge - like tooth. De- 

 vonian. 



Myoconcha Sow. (Fig. 

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

 the right valve ; otherwise resembling Modiolopsis. Carboni- 

 ferous to Cretaceous. 



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

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

 tooth, or edentulous ; adductor scars strong. Jura. 



Modiolodon, JVJiiteavesia, Eurymya, Aristerella, and Prolo- 

 bella Ulrich ; Ordovician ; North America. Goniophora 

 Phillips. Silurian and Devonian. Orthonota Conrad. De- 

 vonian. Orthodesma Hall and AVhitf. Ordovician ; North 

 America. 



Family 39. Mytilidae Fleming. 



Shell equivalve, very ineriuilateral, heteromyarian, slightly 

 gaping, typically dysodont ; area amp)hidetic or none ; ligament 

 usually external, deep-seated ; rarely with an alivincular ^^^^ -og 



internal resilium ; pallial line simple; mantle lobes united MytHusstMacvhSow. Great 

 below the anal siphon, otherwise free; generally byssiferous. Oolite; Minchiiihampton,Eiig- 

 Devoiiian to Recent. ' ' '' 



Modiolopsis inoilwlnris " '"'' 



(Conrad). Ordovician ; Cin- Myoconclm striatula Goldf. Lower Oolite 

 cinnati, Ohio. Vl- Bayeux, Calvados, l/l- 



