Ai!T. 14 FOSSIL MOLLUSKS FROM THE MIOCENE MANSFIELD J 



another specimen, latitude, 104 mm.; altitude, 98 mm., diameter, 

 20 mm. 



Occun^ence. — St. Marys formation, zone 2. U. S. G. S. station 

 3915, Urbanna, Va. (type locality) ; U. S. G. S. station l/476a, La- 

 nexa, Va., lower bed; U. S. G. S. station 1/233, Scotland Wharf, Va., 

 right bank of James River, bed 10 to 16 feet above beach ; U. S. G. S. 

 station 1/540, Schmidts Bluff, right bank of James River, Va., low- 

 est bed ; Mount Folly, right bank of York River, Va. 



THRACIA (CYATHODONTA) DALLI, new Bpecies 



Plate 4, figures 1, 2 



Shell of medium size, thin, translucent, inequilateral — anterior side 

 being longer, right valve larger than left and more inflated. Right 

 valve rounded on anterior side and over the middle of disk and 

 slightly depressed in front of the posterior ridge; left valve slightly 

 depressed medially. Posterior dorsal slope on each valve gradually 

 descending, medially depressed, bordered above by a low ridge 

 extending from the beak to the lower posterior angle, and below by 

 a lower ridge. Posterior dorsal margin nearly straight and oblique; 

 posterior margin truncate; anterior dorsal and basal margins evenly 

 rounded. Sculptured externally by coarse concentric rounded ridges, 

 which are nearly obsolete over the posterior dorsal slope; radially 

 sculptured by faint granulose threads, which are more closely set 

 and more prominent over the posterior dorsal slope. Internally, the 

 external concentric sculpture is reflected. Hinge strongly developed 

 for the genus. 



Dimensions. — Type (Cat. No. 352442, U.S.N.M.), right valve, lati- 

 tude, 34 mm. ; altitude, 25 mm. ; diameter, 9 mm. ; left valve, latitude, 

 33 mm.; altitude (margin broken away), 22 mm.; diameter, 5 mm. 



Type locality. — U. S. G. S. station 1/202, about one-fourth mile 

 northeast of Benns Church, Isle of Wight County, Va. (W. C. 

 Mansfield, collector.) 



The new species is related to Thracia {Gyathodonta) semirugosa 

 Reeve, which species has been reported by Dall " as occurring in the 

 Pliocene marl of the Caloosahatchee River, Fla., and living in the 

 Caribbean Sea. Reeve's species possesses a less oblique posterior 

 dorsal margin and is much less inflated anteriorly than the new 

 species here described. 



The species is named in honor of Dr. AV. H. Dall. 



Occun^ence. — Yorktown formation, zone 2, middle part. 



''Dall, W. II., Wagner Free Inst. Sci. Trans., vol. 3, pt. 6, p. 1526, 1903. 



