52O TRANSACTIONS OF THE WAGNER FREE 



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

 anterior gape are probable evidence of a fibrous byssus. 



FAMILY PERNIDyE. 



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 

 Pterndee, 



Trias to recent fauna. 

 Ex. Perna, Gervillia, Inoceramus, Hornesia, Volviceramus, Crenatu/a, etc. 



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

 reduplicated ligament in the adult. 



FAMILY PTERIIDyE. 



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, Oxytoma, Cassianella, Pseudo- 

 monotis, 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 Pteriid. 



Tertiary to recent fauna, in sponges. 



Ex. Vulsella, 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, 



