136 AMERICAN MARINE CONCHOLOGY. 
throughout, ends diverging, fringed; palpi triangular, narrow ; 
gills plaited, one on each side, with a narrow dorsal border. 
Distribution very extensive, burrowing in sand and mud, 4 to 
110 fathoms. 
1. P. TRILINEATA, Say. Figs. 305-307. 
Journ. Philad. Acad., ii. 261. 1822. 
Shell white, subpellucid, concentrically wrinkled ; wedge-shaped, 
rounded and short before, elongated into a recurved subtruncated 
beak behind. Hinge-margin concavely curved; surface flattened 
and bounded on its edges by two elevated lines from the beaks to 
the rostrated tips. Three or four distinct lines radiate from the 
beaks. Flat valve with two teeth, of which one is shorter and 
more robust than the other; the corresponding cavities in the 
other valve bounded by three tooth-like elevations. Iridescent 
within. 
Height 15, length 30 mill. 
W hole Coast. 
Family SAXICAVIDA. 
Animal symmetrical, oblong. Mantle-lobes united and thick- 
ened in front ; siphons large, elongated, often invested with a thick, 
wrinkled epidermis, united nearly to their ends, the orifices fringed ; 
pedal opening small. Foot small, digitiform, inferior, furnished 
with a byssal groove. 
Perforating stones, or living imbedded in sand and mud. 
Synopsis of Genera. 
Shell oblong, equivalve, valves rugose, gaping, beaks prominent. Hinge 
when young, with two small teeth in each valve, when adult, edentulous ; 
ligament external, more or lessprominent. Muscular impressions strong, 
wide apart; pallial line interrupted, sinuated posteriorly. SAXICAVA. 
Shell transversely oblong, equivalve, gaping at both ends, surface nearly 
smooth or transversely furrowed. Hinge with a single conical tooth in 
each valve, lodged in a cavity of the opposite valve ; ligament short, ex- 
ternal, prominent, attached to strong ridges. Pallial line interrupted, 
with a deep posterior sinus. PANOPABA. 
Shell transversely oblong, equivalve, thick, gaping widely at both ends, 
valves covered with a thick, black, horny epidermis which extends beyond 
theiredges. Hinge callous, edentulous ; ligament large, prominent, exter- 
nal. Muscular impressions wide apart, the hinder elongate ; pallial line 
irregular, strongly marked, the posterior sinus very small. 
CyRTODARIA, 
