628 SYSTEMATIC PALEONTOLOGY 



DREISSENA TIPPANA Conrad 

 Plate XXXVII, Figs. 8-11 



Dreissena tippana Conrad, 1858, Jour. Acad. Nat. Sci., Phila., vol. Hi, p. 



328, pi. xxxiv, fig. 14. 

 Dreissena tippana Meek, 1864, Check List Inv. Fossils, N. A., Cret. and Jur., 



p. 10. 



Description. " Falcate, with distinct lines of growth; front excavated, 

 the margin acutely angular ; the dorsal and posterior margin form a regu- 

 lar curve; base rounded; beaks acute." Conrad, 1858. 



Type Locality. Owl Creek, Tippah County, Mississippi. 



Shell thick, prismatic, strongly falcate in outline, evenly convex, 

 acutely keeled from the umbones to the ventral margin, the carinal angle 

 usually more than 90 and giving to the front view of the double valves a 

 canoe-shaped outline; outline of posterior margin evenly rounded from 

 beaks to base; external surface smooth except for incremental striations 

 and, toward the ventral margin, rather pronounced resting stages; liga- 

 ment groove rather shallow and elongated, hinge edentulous; umbonal 

 septum narrow but quite high ; character of muscle impressions and pallial 

 sinus not preserved ; inner margins simple. 



In Maryland the species is represented chiefly in the form of casts, most 

 frequently of the double valves, to which portions of the brown, prismatic 

 shell substance still adhere, although at some localities perfect specimens 

 have been collected. The form differs quite widely in relative proportions, 

 but it does not seem wise to regard these mutations as of more than indi- 

 vidual import. 



Occurrence. MATAWAN FORMATION. Ulmstead Point, Anne Arundel 

 County. MONMOUTH FORMATION. ? Fredericktown, Cecil County; 

 Brightseat, Brooks estate near Seat Pleasant, 1 mile west of Friendly, 

 Prince George's County. 



Collections. Maryland Geological Survey, U. S. Xational Museum. 



Outside Distribution. Ripley Formation. Exogyra costata zone, 

 Georgia; Eufaula, Alabama; Union and Tippah Counties, Mississippi. 

 Extreme top of zone, Pataula Creek, Georgia ; Chattahoochee River, Ala- 

 bama. 



