INSTITUTE OF SCIENCE, PHILADELPHIA. 553 



resilium external ; area obscure or not defined ; hinge without lateral laminae, 

 with two or three small, usually bifid, radial cardinal teeth in each valve. 



Cretaceous to recent fauna. 

 E.\. Petricola, Choristodon, Naranaio. 



Family GLAUCOMYID^. 



Like Petricola, but with the siphons united to their ends ; the foot small, 

 not byssiferous ; the mantle edges smooth, united, so as to leave but a mode- 

 rate pedal foramen ; dioecious, estuarine or fluviatile in habit. 



Shell with a distinct epidermis ; valves subelongate, with low beaks, more 

 or less gaping behind, smooth, with plain margins ; anterior adductor scar 

 narrow, posterior one rounded, pedal distinct ; ligament and resilium external, 

 parivincular, set upon nymphs ; area obscure or none ; hinge-plate narrow, 

 without lateral laminae ; with three radiating cardinal teeth in each valve, the 

 central bifid, or the left posterior cardinal may be absent. 



Recent. 

 Ex. Glaucomya, Tanysiphon. 



Superfamily TELLINACEA. 



Siphons distinct to their bases, usually long ; pallial line sinuate ; ligament 

 external, seated on nymphs ; hinge with normally an anterior and posterior 

 lateral in each valve, two radial cardinals, of which the anterior is usually 

 bifid and somewhat pedunculated, and the posterior, as well as the laterals, 

 often obsolete. 



Family TELLINID^. 



Gills small, very posterior, not plicate, the outer limb dorsally directed, 

 sometimes without a reflected lamina, or even obsolete ; palpi very large, 

 more or less united behind ; the anterior palps sometimes with antenna-like 

 projections in front ; byssal apparatus obsolete ; foot compressed, short, not 

 grooved, sometimes capable of being flattened ventrally for use as a fulcrum ; 

 mantle margins duplex, with papillose edge, open between the pallial sinus 

 and the anterior adductor ; siphons long, unequal, naked, retractile, with 

 papillose orifices, the branchial without a curtain valve ; the pallial sinus deep, 

 discrepant in the opposite valves ; dicecious, marine. 



Shell substance cellulo-crystalline, with an inconspicuous epidermis ; valves 

 slightly unequal, free, rounded in front, more or less rostrate, oblique and gap- 

 ing behind, compressed, usually with smooth margins, low beaks and variable, 

 chiefly concentric sculpture ; anterior adductor scar larger, frequently irregular ; 

 pedal distinct ; one or two isolated small scars, made by attachments of the mus- 

 cles of the mantle, frequently visible near the ventral posterior termination of 

 the pallial sinus ; resilium embraced in the ligament, subexternal ; area narrow, 

 small, covered with a dark epidermis, or frequently obsolete ; hinge plate narrow, 



