Maryland Geological Survey 497 



Gyrodes petrostis Johnson, 1905, Proc. Acad. Nat. Sci., Phila., p. 21. 

 Gyrodes petrosus Weller, 1907, Geol. Survey of New Jersey, Pal., vol. iv, 

 p. 689, pi. lxxvii, figs. 13-18. 



Description. — " Shell depressed, convex above ; whorls four, rounded ; 

 suture indented ; umbilicus very patulous. Diameter an inch and one- 

 fourth.''— Morton, 1834. 



Type Locality. — Prairie Bluff, Alabama. 



Shell transversely elliptical in outline, depressed ; whorls five in num- 

 ber, spire very low and tapering rapidly to the acute and rather promi- 

 nent apex ; later whorls narrowly tabulated, the shoulder more or less 

 obliquely depressed, acutely angulated at its periphery; body whorl flat- 

 tened in front of the shoulder, rounded medially; external surface smooth 

 excepting for incremental scratches ; sutures distinct, impressed ; aper- 

 ture obliquely ovate in outline, the anterior extremity produced and 

 broader than the posterior, the outer lip thin, sharp, asymmetrically 

 arcuate, inner lip feebly concave; umbilical area large, auriculate, the pit 

 at its posterior extremity; umbilical margin more or less acutely angu- 

 lated. 



Gyrodes petrosus (Morton) is well characterized by the oblique com- 

 pression of the posterior half of the whorl, the flattening and depression of 

 the whorl in front of the suture, the angulated umbilical keel and the very 

 much produced aperture. 



Occurrence. — Matawax Formation, Post 218, Chesapeake and Dela- 

 ware Canal, Delaware. Monmouth Formation. ? Bohemia Mills, 

 Cecil County ; Brightseat, Brooks estate near Seat Pleasant, railroad cut 

 1 mile west of Seat Pleasant, 1 mile west of Friendly, 2 miles southwest 

 of Oxon Hill, Prince George's County, Maryland. 



Collections. — Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences, New Jersey Geological Survey, U. S. National Museum. 



Outside Distribution. — Matawan Formation. Meivhantville clay marl, 

 Wenonah sand, New Jersey. Monmouth Formation. Navesink marl, 

 Eed Bank sand, New Jersey. Ripley Formation. Exogyra costata zone, 

 Union and Tippah counties, Mississippi. Selma Chalk. Exogyra costata 

 zone, Wilcox County, Alabama ; east-central Mississippi. 



