520 TRANSACTIONS OF THE WAGNER FREE 



developed an anterior siphon like Leptoiiacea, and the serrations of part of the 

 anterior gape are probable evidence of a fibrous byssus. 



Family PERNIP.-E. 



Shell submytiliform with a broad posterior wing ; monomyarian, the an- 

 terior adductor absent in the adult ; inequivalve, edentulous, with a serial 

 multivincular ligament ; byssiferous, with a moderate gape, or none ; gills re- 

 ticulate, united to each other and to the mantle ; soft parts generally as in 

 Pteriidm. 



Trias to recent fauna. 

 Ex. Perna, Gervillia, Inoceranms, Hornesia, Volviceramus, Crenaiula, etc. 



This group differs from the next chiefly by its edentulous hinge and 

 reduplicated ligament in the adult. 



Family PTERIID^. 



Shell aviculoid, bialate, monomyarian, inequivalve, with an alivincular 

 ligament; the byssus issuing by a notch in the smaller valve; ventricle ven- 

 tral to the rectum, with anterior and posterior aortas ; gills filamentary or 

 imperfectly reticulate, with direct and reflected laminae attached to the mantle, 

 but the tips behind the adductor floating free; foot subcylindrical, small, 

 grooved ; anal end of rectum free, erectile ; the young dimyarian, sometimes 

 passing through a distinct nepionic stage. 



Lower Silurian to recent fauna. 



Ex. Pteria, Meleagrina, Philobrya, Malleus, Monolis, Oxyloma, Cassianella, Pseudo- 

 inonotis, Aucella, etc. 



The gills differ in different species, the shallow-water forms often being 

 more or less organically reticulate, while the abyssal forms have the filaments 

 free, or connected only by cilia, like the abyssal arks. 



Family VULSELLID.^. 



Shell ostreiform, not alate, monomyarian, edentulous, inequivalve, with 

 an alivincular ligament ; without a byssus ; ventricle embracing the rectum ; 

 gills filibranchiate with ciliary junctions; foot grooved, angularly bent ; other- 

 wise as in Pteriidce. 



Tertiary to recent fauna, in sponges. 

 Ex. Vtdsella, Vulsellina. 



A degraded type which has become specialized through commensalism. 



Superfamily OSTRACEA. 



Shell degenerate, sessile, inequivalve, generally edentulous, ala; obsolete; 

 with a subnacreous or porcellanous inner, and prismatic outer layer ; epidermis 

 inconspicuous ; area amphidetic, ligament alivincular ; foot and byssus absent, 



