MARINE MOLLUSCA OF THE UNITED STATES. 123 



SYNOPSIS OF FAMILIES. 



A. Animal provided with siphons. 

 a. Pallial impression sinuated 



PHOLADIM. Shell free; valves equal, gaping at both ends, 

 thin, white, brittle, armed in front with rasp-like imbrications, 

 without hinge-teeth, and strengthened externally by accessory 

 valves ; hinge plate reflexed over the beaks, and furnished with a 

 long, curved, tooth-like process beneath each ; anterior muscular 

 impression on the hinge-plate; pallia! sinus very deep; living 

 perpendicularly in holes in rock or sand. 



GASTROCH^ENID^J. Animal symmetrical, elongated ; with two 

 long contractile siphons posteriorly, united nearly to their extre- 

 mities, which are fringed with cirrated orifices. Shell : Valves 

 thin, gaping, edentulous, ligament external, adductor impressions 

 two, pallial line sinuated ; contained within a shelly tube, both 

 valves free, or one or both valves cemented to its walls. 



Burrowing in wood, stone, sand or mud at low water mark, and 

 lining the burrow with a calcareous tube. 



TEREDID^E. Animal worm-like ; siphons furnished at their ex- 

 tremities with two shelly styles or palletes ; shell contained in a 

 shelly tube, globose, its valves trilobate, gaping anteriorly and be- 

 hind, without hinge-teeth or accessory valves ; hinge-plate reflexed 

 over the beaks, and furnished with a long tooth-like process be- 

 neath each ; living in burrows in wood, which they line with a 

 calcareous tube. 



ANATINID^E. Often inequi valve, thin ; interior nacreous ; sur- 

 face granular ; ligament external, thin ; cartilage internal, placed 

 in corresponding pits and furnished with a free ossicle ; muscular 

 impressions faint, the anterior elongated ; pallial line usually 

 sinuated. 



SAXICAVIDJE. Shell equi valve, solid, gaping at each end ; hinge- 

 teeth rudimentary; cartilage external, thick, prominent; pallial 

 impression irregular, sinuated posteriorly ; perforating stones or 

 imbedded in sand or mud. 



MYID^E. Shell thick, strong, opaque, porcellanous, gaping pos- 

 teriorly, valves usually unequal, covered with a wrinkled epider- 

 mis; hinge simple, toothless, but with a hollow process of the 

 beak in one valve, containing the ligament. 



SOLENID^E. Shell elongated, gaping at the ends ; ligament ex- 



