148 STSTEIIATIC PALEOXTOLOGT 



ration of the wliorls, and if it were not for the great number of inter- 

 mediate types one might readily establisli several independent species. 



Some specimens have nearly equally rounded whorls with evenly 

 spaced spiral lines, while others are prominently carinated toward the 

 base of the whorl and the spiral threads are irregularly spaced, varying 

 in jDrominence and number. 



A variety named posfmortoni by Harris is characterized by its rather 

 smaller size, plainer surface, and extremely sharp basal carina. Figure 

 5 represents this form. 



Length, 130 mm.; width, 33 mm. 



Occurrence. — AqriA Foematiox. Fort Washington, 1 mile northeast 

 of Piscatawa}^, Korthwest of Piscataway ( W. of Tinkers Cr.), Brooks Es- 

 tate near Seat Pleasant, Gtymont, Liverpool Point, Clifton Beach, 1 

 mile southeast of Mason Springs, Aqnia Creek, Potomac Creek, Upper 

 Marlboro, Sheckels Farm near South Eiver, 1 mile west of Hardesty, 

 Eolph's Landing. 



Collections. — Maryland Geological Survey, Johns Hopkins University, 

 U. S. National Museum, Philadelphia Academy of Natural Sciences. 



TuEEiTELLA HUMEEOSA Conrad. 

 Plate XXA'II, Figs. 1, la. 



Turritella humerosa Conrad, 1835, Traus. Geol. Soc, Penn., p. 340, pi. xiii, fig. 3. 

 Turritella humerosa H. C. Lea, 1848, Proc. Acad. Nat. Sci., Phila., vol. iv, p. 107. 

 Tarritella humerosa Conrad, 186.5, Amer. Jour. Conch., vol. i, p. 33. 

 Turritella Jmraerosa Conrad, 1866, Smith. Misc. Coll. (200), p. 11. 

 Turritella humerosa Harris, 1894, Amer. Jour. Sci., ser. iii, vol. xlvii, p. 303. 

 Turritella humerosa Clark, 1895, Johns Hopkins Univ. Circ, vol. xv, p. 4. 

 Turritella humerosa Clark, 1896, Bull. 141, U. S. Geol. Survey, p. 70, pi. xiv, fiu-. 1. 

 Tarritella Jiumerosa Harris, 1899, Bull. Amer. Pal., No. 11, p. 75, figs. 5, 6, 7. 



Description. — " Shell turreted, subulate; whorls with fine regular re- 

 volving striae; an obtuse slight elevation on the summit, a shallow 

 groove at the base of each." Conrad, 183.5. 



Turritella humerosa, although not an uncommon species, is far less 

 frequent than T. mortoni. It is very readily separated from the latter 

 by its flat whorls and prominent subsutural carina, which makes the 

 whorls of nearly equal diameter above and below. 



