154 SYSTEMATIC PALEONTOLOGY 



inflected forward and prominent; some of the varices are more promi- 

 nent than otherS;, and in a few instances are partially divided by a 

 central groove; aperture nearly round." Clark, 1896. 



Some specimens show a large umbilicus, in others it is absent. 



Length, 22 mm.; width, 14 mm. 



Occurrence. — Aquia Foemation. Aquia Creek, Potomac Creek. 



Collection. — U. S. National Museum, Philadelphia Academy of Natural 

 Sciences. 



SCALA POTOMACENSIS U. Sp. 



Plate XXVIII, Fig. 10. 



Description. — Shell elongate, moderately slender, eight-whorled; 

 mouth round; base with numerous, closely-set, revolving lines and folds 

 radiating to the varices; whorls closely-set; moderately and uniformly 

 convex; varices low, obtuse, twenty-five on the body whorl, and about as 

 many on the whorls of the spire, stronger than ordinary, and occurring 

 at irregular intervals; raised revolving threads, about 30 in number, 

 alternating and doubly alternating in strength, run between without 

 crossing the varices. 



Length, 20 mm.; width, 8 mm. 



Occurrence.- — Aquia Fokmation. 2 miles below Potomac Creek. 



Collection. — Marjdand Geological Survey. 



ScALA SESSiLis Conrad. 

 Plate XXVIII, Fig. 11. 



Scala sessilis Conrad, 1833, Fossil Shells of the Tertiary, No. 4, p. 45. 



Description. — " Subulate, with rather thick longitudinal costae, and 

 minute crowded spiral lines; whorls nine, sessile or contiguous; base of 

 the body whorl carinated." Conrad, 1833. 



Occurrence. — Aquia Foemation. 2 miles below Potomac Creek. 



Collection. — Maryland Geological Survey. 



Scala caeinata Lea. 

 Plate XXVIII, Fig. 12. 



Scala carinata Lea, 1833, Coutrib. to Geol., p. 116, pi. iv, fig, 103. 

 Description. — " Shell conical, closely ribbed, carinate on the inferior 

 part of the last whorl; substance of the shell thick; spire elevated, 



