MARYLAND GKOLOOICAL SURVEY 155 



})(iiiitc(l; ril)s aljiiut twchc, laiiiflliruiiii ; uliurls six, rmindc'd; mouth 

 I'oiiiiil."' IjViX, 1833. 



Occurrence. — Aquia Foit.MATiox. 2 miles below Potomac Creek, Up- 

 per Marlboro. 



Colleclion. — Marylaiul (Jeolugical Survey. 



Supcrtamily GYMNOGLOSSA. 

 Family PYRAMIDELLIDAE. 



Genus TURBONILLA Risso. 

 Subgenus PYRGISCUS Philippi. 



TURBONILLA POTOMACENSIS n. sp. 



Plate XXIX, Fig. 1. 



Description. — Shell stout; whorls eight, moderately convex; about 

 twenty-five slightly curved, longitudinal ribs on the body whorl, crossed 

 by eight, impressed, revolving lines, which are deepest between the ribs; 

 base of shell with impressed, revolving lines, and faint, radiating ribs; 

 mouth ovate. 



Length. 10 mm.; width, 4 mm. 



Occurrence. — Aquia Formation. Potomac Creek, 2 miles below Po- 

 tomac Creek. 



Collection. — Maryland Geological Survey. 



Genus TUBA Lea. 



Tuba marylaxdica n. sp. 

 Plate XXIX, Fig. 2. 



Description. — Shell elongate; whorls seven, strongly convex, covered 

 with eight raised revolving threads, with narrower interspaces; threads' 

 strongly beaded by intersections with raised longitudinal lines. 



Length, 12 mm.; width, 5 mm. 



Occurrence. — Xakjemoy Foematiox. Popes Creek. Aquia Foeaia- 

 TiON. Mouth of Paspotansa Creek, 1 mile southeast of Mason Springs. 



Collection. — Maryland Geological Survey. 



