INSTITUTE OF SCIENCE, PHILADELPHIA. 555 



Superfamily SOLENACEA. 



Dwellers in soft bottom, narrow, elongated, modified for burrowing, with 

 anterior and posterior ends both gaping ; foot elongated, distally modified to 

 serve as a piston or stilt within the burrow ; aorta with postventricular dila- 

 tion ; hinge without lateral laminae. 



Family SOLENID^. 



Gills plicate, prolonged, normal, with the lamina; appendiculate in front, 

 arising between the united palpi, behind the foot joined to the siphonal septum, 

 but not to each other; palpi large, united and produced into points behind, 

 simple in front; foot long, subcylindric, clavate, often with a flattened oblique 

 anterior surface, serving as a stilt; mantle lobes united ventrally, rarely with a 

 small ventral foramen, more or less papillose; siphons usually papillose, naked, 

 variable in length and in amount of separation ; adductors thin, long, flat, dor- 

 sally extended; dioecious, usually marine or estuarine. 



Shell substance as in Tellina, but the external layer showing its cellular 

 structure more clearly; with a pronounced epidermis; valves equal, free, 

 usually truncate at both ends, and more or less inequilateral, with low beaks, 

 smooth margins, smooth or feebly sculptured, not rostrate ; adductor scars 

 narrow, elongate, dorsally distributed, pedal distinct ; pallial sinus small in 

 species, with anterior umbones and vice versa ; ligament and resilium external, 

 parivincular, seated on nymphs ; area obscure or none ; hinge-plate hardly 

 developed ; hinge often with a thickened ray crossing the valves and serving 

 as a buttress; cardinals varying from one to four in each valve, usually a single 

 slender radial laminar cardinal in the right, and two in the left valve ; with or 

 without one or two placed parallel with the hinge margin, simulating laterals ; 

 radial teeth usually more or less pedunculated, rarely bifid. 



Devonian to recent fauna. 

 Ex. PalcEosolen, Leptosolen, Ensis, Solen, Cultellus, Pharus, Pharella, Siliqua, Tagelus, 

 Soleciirtus, Novaculina. 



The Silurian forms, heretofore referred to this family, do not seem to be- 

 long to it, but Palcsosolen, Hall, seems almost indistinguishable from some 

 modern forms. The species of this family are mostly much modified for a 

 special mode of life, hence the variability in certain features, such as the si- 

 phons, foot, and fermature of the mantle lobes. Novaculina has been natural- 

 ized in fresh water, and Tagebis appears to prefer estuaries. 



The group has long been associated with the Myacea on account of their 

 common adaptive characters, but there can be little doubt that its situation here 

 is more in accordance with its relations. 



Superfamily MACTRACEA. 



Resilium internal, seated on chondrophores ; left cardinal tooth bifid, fit- 

 ting below the two right cardinals, which are more or less joined together 



