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. 305-30T. 

 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. 



Whole Coast. 



Family SAXICAVIDJE. 



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 less prominent. 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. PANOP^EA. 



Shell transversely oblong, equivalve, thick, gaping widely at both ends, 

 valves covered with a thick, black, horny epidermis which extends beyond 

 their edges. 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. 



