5 28 TRANSACTIONS OF THE WAGNER FREE 



The heavier forms 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. 



These forms, not including some which have been referred to the family, 

 appear to be early prototypes of the Mytilidtz, from which they differ chiefly in 

 those characters which are common to most of the ancient types, such as the 

 subequality of the adductor scars and their more dorsal situation. The recent 

 Idas is very similar. 



FAMILY MYTILID^E. 



Shell equivalve, very inequilateral, heteromyarian, slightly gaping, typi- 

 cally dysodont ; area amphidetic or none ; ligament usually external, deep- 

 seated ; rarely with an alivincular internal resilium ; pallial line simple ; ventri- 

 cle with an anterior aorta ; gills attached distally and dorsally, usually with a 

 reflected limb to the filaments ; mantle lobes united below the anal siphon, 

 otherwise free ; a byssus generally present. 



Devonian to recent fauna. 



Ex. Naiadites, Myalina, Hoplomylilus, Mytilus, Modiola, Lithophagus, Modiolaria, Cren- 

 ella, Byssanodonta, Stavelia, Dacrydium, Myrina. 



The pedal protractors in Myalina are set upon a myophore; in Dacryd- 

 ium there is a small subumbonal internal resilium ; though the shell is 

 smooth, the inner faces of the dorsal margin retain transversely rugose areas 

 like those of Dimya. As in other cases some of the abyssal species have no 

 reflected limb to the gill filaments. The shell of Stavelia, as a whole, is 

 spirally twisted like some Paleoconcha. The shell substance of Lithophagus 

 is penetrated by minute tubules. 



FAMILY DREISSENSIIDyE. 



Shell mytiliform, equivalve, of prevailingly prismatic substance ; area 

 linear, amphidetic ; ligament subinternal ; anterior adductor and pedal protrac- 

 tors inserted on a myophoric septum ; mantle lobes united to form anal and 

 branchial siphons, and also ventrally with a pedal opening, pallial line usually 

 simple ; gills reticulate, attached to each other behind the foot, otherwise free, 

 distally ; the rest of the anatomy like Mytilus. 



Tertiary to recent fauna. 

 Ex. Dreissensia, Congeria, Mytilopsis, ? Septifer, ? Dreissensiomya. 



The tertiary Dreissensiomya has a distinct pallial sinus, the only excep- 

 tion in the Mytilacea to the usual rule, though there is sometimes a faint sinu- 

 osity in Modiolaria. In the American Mytilopsis the myophore for the pedal 

 protractor is distinct from that which supports the anterior adductor. 



FAMILY MODIOLARCID^E. 



Shell modioliform, small, equivalve, free, with a narrow byssal gape ; area 

 obscure or none, the space just in front of the beaks usually impressed ;' liga- 



