MARINE MOLLUSCA OF THE UNITED STATES. 123 
- SYNOPSIS OF FAMILIES. 
A. Animal provided with siphons. 
a. Pallial impression sinuated. 
PHoLapips. 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; pallial sinus very deep; living 
perpendicularly in holes in rock or sand. 
GastrocHaNID#. 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. 
Trrepips#. Animal worm-like; siphons furnished at their ex- 
tremities with two shelly styles or palletes; shell contained ina 
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 along tooth-like process be- 
neath each; living in burrows in wood, which they line with a 
calcareous tube. 
ANATINIDA. 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. i 
Saxtcavip#. 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. 
Myip#. 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. 
Sonenipa. Shell elongated, gaping at the ends ; ligament ex- 
