INSTITUTE OF SCIENCE, PHILADELPHIA. 543 



Superfamily RUDISTAE. 



Chatnaceain whicli the spirality of the valves has been lost, the area and 

 ligament vertically submerged, and the dorsal margins recurved over them so 

 as to bring the ligament into a subcentral position, above the teeth but far below 

 the dorsal margin, where it finally becomes obsolete. 



The teeth, no longer forming a hinge but rather a clithrurn, are specially 

 modified for the vertical motion of the operculiform left valve, in which rota- 

 tion is prevented by the projection of the modified teeth into deep sockets in 

 the fixed valve ; the latter conical, thick ; pallial line simple, enclosing the whole 

 cavity ; shell structure specialized in two very different layers ; sessile, marine. 



Family RADIOLITID^E. 



Shell substance with the external layer thick, prismatic, the internal thin, 

 cellulo-crystalline (frequently destroyed in fossilization) ; valves very unequal, 

 the ligamentary subsidence usually marked ; free valve with two projections 

 and two somewhat irregular myophores ; fixed valve with one myophore and 

 two sockets ; summit of the valves submarginal in the young, subcentral in the 

 adult. 



Cretaceous. 

 Ex. Radiolites, Biradiolites. 



The visible submersion of the ligament in some Radiolites enables us to 

 understand how the conditions precedent for the HippuritidcB might have been 

 brought about. 



Family HIPPURITID^. 



Shell substance of two layers, the external porous, grooved and punctate, 

 the inner lacunary and prismatic ; exterior with sutures corresponding to an 

 " anal " and " branchial " inflection, and sometimes with a ligamentary suture ; 

 clithrum formed of two processes in the free valve, the adductors attached to 

 myophores; fixed valve with one thin laminar process, the adductor scars ex- 

 cavated, the anterior adductor duplex, forming distinct scars. 



Cretaceous. 

 Ex. Hippurites, Arnaudia. 



Superfamily LUCINACEA. 



Shell with the anterior adductor scar narrower, produced ventrally ; pos- 

 terior scar shorter, rounded ; pallial line simple ; foot elongated, subclavate ; 

 hinge feeble, teeth radial, often obsolete. 



Family TANCREDnD.<E? 



Shell donaciform, equivalve, with an external ligament; the margin of the 

 valves entire; hinge with posterior and anterior lateral laminae, the latter in- 



