544 Systematic Paleontology 



in fresh specimens, arranged in an asymmetric but continuous series; 

 anterior portion of the series feebly concave, the teeth subequal, approxi- 

 mately twelve in number; posterior portion of series feebly convex and 

 subparallcl to the dorsal margin, eighteen to twenty-two in number, the 

 half-dozen or so beneath the umbones smaller than the others ; adductor 

 muscles two in number, subequal, situated near the median horizontal, 

 the anterior slightly higher than the posterior; pallial line simple, distinct, 

 rather remote from the base ; inner margins non-crenate. 



Dimensions. — Altitude 8.6 mm. ; latitude 8.8 mm. ; diameter of double 

 valves 3.8 mm. 



Type Locality. — Friendly, Prince George's County. 



This species is one of the most abundant of the smaller bivalves of 

 Prince George's County. 



Occurrence. — Monmouth Formation. — Brightseat, Brooks estate near 

 Seat Pleasant, Friendly, 1 mile west of Friendly, and MciSTeys Corners, 

 Prince George's County. 



Collection. — Maryland Geological Survey. 



B. Schizodonta 



Superfamily PTERIACEA 



Family PINN1DAE 



Genus PINNA Linne 

 [Systema Naturae, 1758, ed. x, p. 707] 



Type. — Pinna rudis Linne. 



Shell thin, fragile, byssiferous, with a fibrous external layer, nacreous 

 within; valves inequilateral, widely gaping posteriorly, trigonal or sub- 

 cuneiform in outline, non-auriculate, often mesially rostrate longitudi- 

 nally ; external surface usually more or less scaly, umbones terminal, ante- 

 rior, acute; ligament elongated, lodged in a deep groove, hinge line 

 straight, edentulous, muscle scars very unequal in size, the anterior small 

 and high up beneath the beaks, the posterior relatively very large and sub- 

 central in position ; pallial line simple. 



Etymology: Pinna, a feather. 



