MARINE MOLLUSCA OF THE UNITED STATES. 123 



Synopsis op Families. 



A. Animal provided loith siphons, 

 a. Pallial impression sinuated 



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

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

 witliout hinge-teeth, and strengthened externally by accessory 

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

 long, curved, tooth-like pi'ocess beneath each ; anterior muscular 

 impression on the hinge-plate ; pallial sinus very deep ; living 

 perpendicularly in holes in rock or sand. 



Gastroch^nid^. Animal symmetrical, elongated ; with two 

 long contractile siphons posteriorly, united nearly' to their extre- 

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

 thin, gaping, edentulous, ligament external, adductor impressions 

 two, pallial line sinuated ; contained within a shell}^ 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^. Animal worm-like ; siphons furnished at their ex- 

 tremities with two shelly stjdes 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^. Often inequivalve, 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. 



Saxicavid^. Shell equivalve, 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.^. Shell thick, strong, opaque, porcellanous, gaping pos- 

 teriorl3% 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. 



SoLENiDyE. Shell elongated, gaping at the ends ; ligament ex- 



